This explains why Ramakrishna Paramahamsa was able to follow Hinduism, Christianity and Islam and reach the same state of realization at the end.
Advaita Vedanta does not say that all religions are true. It is Sri Ramakrishna and Swami Vivekananda who say that. I have posted 2 quotes of Sri Ramakrishna and 1 of Swami Vivekananda to explain what they mean by 'all religions are true'.
MUSICIAN: “Sir, what is the way to realize God?”
MASTER: “Bhakti is the one essential thing. To be sure. God exists in all beings. Who, then, is a devotee? He whose mind dwells on God. But this is not possible as long as one has egotism and vanity. The water of God’s grace cannot collect on the high mound of egotism. It runs down. I am a mere machine.
(To Kedar and the other devotees) “God can be realized through all paths. All religions are true. The important thing is to reach the roof. You can reach it by stone stairs or by wooden stairs or by bamboo steps or by a rope. You can also climb up by a bamboo pole. “You may say that there are many errors and superstitions in another religion. I should reply: Suppose there are. Every religion has errors. Everyone thinks that his watch alone gives the correct time. It is enough to have yearning for God. It is enough to love Him and feel attracted to Him. Don’t you know that God is the Inner Guide? He sees the longing of our heart and the yearning of our soul. Suppose a man has several sons. The older boys address him distinctly as ‘Baba’ or ‘Papa’, but the babies can at best call him ‘Ba’ or ‘Pa’. Now, will the father be angry with those who address him in this indistinct way? The father knows that they too are calling him, only they cannot pronounce his name well. All children are the same to the father. Likewise, the devotees call on God alone, though by different names. They call on one Person only. God is one, but His names are many.”
The Gospel of Sri Ramakrishna, Chapter 4, Advice to Householders, August 13, 1882
(To the Goswami) With sincerity and earnestness one can realize God through all religions. The Vaishnavas will realize God, and so will the Saktas the Vedantists, and the Brahmos. The Mussalmans and Christians will realize Him too. All will certainly realize God if they are earnest and sincere.
“Some people indulge in quarrels, saying, ‘One cannot attain anything unless one Worships our Krishna’, or, ‘Nothing can be gained without the worship of Kali, our Divine Mother’, or, ‘One cannot be saved without accepting the Christian religion.’ This is pure dogmatism. The dogmatist says ‘My religion alone is true, and the religions of others are false.’ This is-a bad attitude. God can be reached by different paths.
“Further, some say that God has form and is not formless. Thus they start quarrelling. A Vaishnava quarrels with a Vedantist.
“One can rightly speak of God only after one has seen Him. He who has seen God knows really and truly that God has form and that He is formless as well. He has many other aspects that cannot be described.
“Once some blind men chanced to come near an animal that someone told them was an elephant. They were asked what the elephant was like. The blind men began to feel its body. One of them said the elephant was like a pillar; he had touched only its leg. Another said it was like a winnowing-fan; he had touched only its ear. In this way the others, having touched its tail or belly, gave their different versions of the elephant. Just so, a man who has seen only one aspect of God limits God to that alone. It is his conviction that God cannot be anything else.
(To the goswami) “How can you say that the only truth about God is that He has form? It is undoubtedly true that God comes down to earth in a human form, as in the case of Krishna. And it is true as well that God reveals Himself to His devotees in various forms. But it, is also true that God is formless; He is the Indivisible Existence-Knowledge-Bliss Absolute. He has been described in the Vedas both as formless and as endowed with form. He is also described there both as attributeless and as endowed with attributes.
Do you know what I mean? Satchidananda is like an infinite ocean. Intense cold freezes the water into ice, which floats on the ocean in blocks or various forms. Likewise, through the cooling influence of bhakti, one sees forms of God in the Ocean of the Absolute. These forms are meant for the bhaktas, the lovers of God. But when the Sun of Knowledge rises, the ice melts; it becomes the same water it was before. Water above and water below, everywhere nothing but water. Therefore a prayer in the Bhagavata says: ‘O Lord, Thou hast form, and Thou art also formless. Thou walkest before us, O Lord, in the shape of a man; again, Thou hast been described in the Vedas as beyond words and thought.’
But you may say that for certain devotees God assumes eternal forms. There are places in the ocean where the ice doesn’t melt at all. It assumes the form of quartz.”
The Gospel of Sri Ramakrishna, Chapter 8, The Master’s Birthday Celebration at Dakshineswar, March 11, 1883
For, you see, in three ways man perceives God. At first the undeveloped intellect of the uneducated man sees God as far away, up in the heavens somewhere, sitting on a throne as a great Judge. He looks upon Him as a fire, as a terror. Now, that is good, for there is nothing bad in it. You must remember that humanity travels not from error to truth, but from truth to truth; it may be, if you like it better, from lower truth to higher truth, but never from error to truth. Suppose you start from here and travel towards the sun in a straight line. From here the sun looks only small in size. Suppose you go forward a million miles, the sun will be much bigger. At every stage the sun will become bigger and bigger. Suppose twenty thousand photographs had been taken of the same sun, from different standpoints; these twenty thousand photographs will all certainly differ from one another. But can you deny that each is a photograph of the same sun? So all forms of religion, high or low, are just different stages toward that eternal state of Light, which is God Himself. Some embody a lower view, some a higher, and that is all the difference. Therefore, the religions of the unthinking masses all over the world must be, and have always been, of a God who is outside of the universe, who lives in heaven, who governs from that place, who is a punisher of the bad and a rewarder of the good, and so on. As man advanced spiritually, he began to feel that God was omnipresent, that He must be in him, that He must be everywhere, that He was not a distant God, but dearly the Soul of all souls. As my soul moves my body, even so is God the mover of my soul. Soul within soul. And a few individuals who had developed enough and were pure enough, went still further, and at last found God. As the New Testament says, “Blessed are the pure in heart, for they shall see God.” And they found at last that they and the Father were one.
The Complete Works of Swami Vivekananda, Volume 4, Lectures and Discourses, Christ the Messenger
If you have read the quotes then you will see that the statement 'all religions are true' does not mean all religions are correct. In fact Ramakrishna thought that there is error in every religion. Moreover some religions are better than others. The presence of errors does not mean one can say that my religion is true and other religions are false. The reason is the God does not care about theological views but only wants purified mind.
Ramakrishna thought that Saguna Brahman is as real as Nirguna Brahman. He definitely differs from Shankaracharya on this issue. Let me use an analogy of the noon Sun and the evening Sun to describe the difference. The noon Sun (in a cloudless sky) is bright yellow and felt to be hot while the evening Sun is red and is kinder and gentler. The difference is that the rays of evening Sun have to travel through lot more atmosphere than the noon Sun. Shankaracharya says that the evening Sun is the noon Sun plus attributes and that the noon Sun is the real entity. Ramakrishna is saying that both the noon and the evening Sun are Sun and equally real.
The problem with your question is a fundamental misunderstanding of first Saguna Brahman and Nirguna Brahman. There is only One. One Brahman. There are not two. Sa-guna (with attributes/qualities) Brahman and Nir-guna (without attributes/qualities) Brahman are the same Brahman. Not two. One.
We, as individuals, as parts of this sensual universe, can only think and perceive in terms of qualities - gunas. We cannot perceive, conceive, or make mental constructs of anything which is beyond sensual qualities. We cannot come to a mental construct as to 'what' Brahman 'is'. For us to try and make a mental construct of Brahman, we try to make a construct of a 'Being' with supreme qualities. This mental construct that we make is Saguna Brahman - Iswara. We are all human, so many of the constructs each of us makes will have some likeness to what others will conceive as a Supreme Being. But we all have a different view. When I look up at the moon, I will see a different moon than you will see. Besides viewing it from different angles, I am old, so my vision my night vision is not what it was when I was young. I cannot see all the stars around the moon, and it appears dimmer than when I was young. I also need glasses, and old age just makes the eyesight poorer. The Reality is distorted by the medium through which each one of us sees it. Just as each one of our views of the moon is distorted by our position and individual limitations, so is our view of Brahman. But it is the same moon. Krishna in Gita Chapter 7 says (Swami Nikhilananda translator):
Whatever may be the form a devotee seeks to worship with faith--in that form alone I make his faith unwavering.
Not knowing my supreme Nature, immutable and transcendent, foolish men that that I, the Unmanifest, am endowed with a manifest form.
Veiled by My maya born of the gunas, I am not revealed to all. This deluded world knows Me not as the unborn and eternal.
We are all of limited consciousness and can only think of Brahman when we assign qualities. Brahman without qualities is beyond human consciousness. We all, when we try and think of Brahman, can only think of Brahman with qualities. 'Saguna' Brahman.
In The Gospel of Sri Ramakrishna, Ramakrishna says (Chapter 29):
But I say that we are all calling on the same God. Jealousy and malice need not be. Some say that God is formless, and some that God has form. I say, let one man meditate on God with form if he believes in form, and let another meditate on the formless Deity if he does not believe in form. What I mean is that dogmatism is not good. It is not good to feel that my religion alone is true and other religions are false. The correct attitude is this: My religion is right, but I do not know whether other religions are right or wrong, true or false. I say this because one cannot know the true nature of God unless one realizes Him.
Do you know what the truth is? God has made different religions to suit different aspirants, times, and countries. All doctrines are only so many paths; but a path is by no means God Himself. Indeed, one can reach God if one follows any of the paths with whole-hearted devotion. Suppose there are errors in the religion that one has accepted; if one is sincere and earnest, then God Himself will correct those errors. Suppose a man has set out with a sincere desire to visit Jagannath at Puri and by mistake has gone north instead of south; then certainly someone meeting him on the way will tell him: 'My good fellow, don't go that way. Go to the south.' And the man will reach Jagannath sooner or later.
If there are errors in other religions, that is none of our business. God, to whom the world belongs, takes care of that. Our duty is somehow to visit Jagannath. (To the Brahmos) The view you hold is good indeed. You describe God as formless. That is fine. One may eat a cake with icing, either straight or sidewise. It will taste sweet either way.
Religion is in realization, it is not what you believe or what I believe. God doe not belong to a religion. Religions belong to God. As Swami Vivekananda said (Complete Works, Vol. 1):
The aim is to get rid of nature's control over us. This is the goal of all religions. Each soul is potentially divine. The goal is to manifest this divinity within, by controlling nature, external and internal. Do this either by work, or worship, or physic control or philosophy--by one or more or all of these--and be free. This is the whole of religion. Doctrines, or dogmas, or rituals, or books, or temples, or forms, are but secondary details.
In Advaita does “all religions are true” mean that the Saguna deity you pass through towards Nirguna Brahman doesn't matter?
It might be better to say that all religions have truthful content; since as limited beings, its not possible for us to fully comprehend what enlightment is.
The simplest stage to get to is dvaita, which is dualism; Advaita, is much, much more difficult, I expect.
I once, after several nights of not sleeping, slept, and in the morning, before I awoke, had a distinct feeling as though I was seeing daylight being poured into my mind - and then I woke up; I hadn't dreaming, as dreams have a very different feeling; moreover, draems tend to be quite complex; this was very, very simple; just darkness turning into light as light flooded in; personally speaking, I wouldn't consider myself as enlightened in the metaphysical sense; but I am merely observing what happened ... I found it quite astonishing and for a whole day I was elated - but that is another story.