First thing you have to study about the oldest religion, As we all know it is "Sanatan Dharama", and Hinduism is not different religion, it itself Sanatan Dharama.
Now as per the researching, we know that Hinduism is oldest once, so, but obvious,It is a root of all other religions.
Now According to Bhagavad Gita, There are total three types of Faith in Person,
(1)Sattvic Faith (Mode of Goodness)
(2)Rajasic Faith (Mode of Passion)
(3)Tamasic Faith (Mode of Ignorance)
Now we look out the Bhagavad Gita Chapter 17
ye śāstra vidhim utsṛjya yajante śraddhayānvitāḥ |
teṣāṃ niṣṭhā tu kā kṛṣṇa satvam āho rajas tamaḥ ||17.1 ||
Arjuna said :What is the position O Krishna of those who disregard the injunctions of the Shastra, yet
worship with conviction (śraddhā)? Is it Sattva, Rajas or Tamas'?
śrī bhagavān uvāca:
trividhā bhavati śraddhā dehināṃ sā svabhāvajā |
sātvikī rājasī caiva tāmasī ceti tāṃ śṛṇu ||17.2 ||
The Blessed Lord said: Threefold is the conviction of embodied beings, inherent in their own nature, classified as
Sattvic, Rajasic and Tamasic — listen to the teaching about it.
sattvānurūpā sarvasya śraddhā bhavati bhārata |
śraddhāmayo’yaṃ puruṣo yo yacchraddhaḥ sa eva saḥ ||17.3 ||
The conviction of each person, O Arjuna, is in accordance with one's character. A personconsists of conviction, and whatever one's conviction is, that verily one is
yajante sātvikā devān yakṣa rakṣāṃsi rājasāḥ |
pretān bhūtagaṇāṃścānye yajante tāmasā janāḥ ||17.4 ||
The Sattvic types worship the gods. The Rajasic types worship Yakshas and Rakshasas, and the
others, classified as Tamasic, worship Pretas and the host of Bhūtas.
Now You have to focus on this Sloka of Bhagavad-Gita.
aśāstra vihitaṃ ghoraṃ tapyante ye tapo janāḥ |
dambhāhaṅkāra saṃyuktāḥ kāma rāga balānvitāḥ ||17.5 ||
Those who practice severe self-discipline (tapas) not enjoined by the Śāstras, through
ostentation and egoism, and impelled by the force of passion and attachment;
karṣayantaḥ śarīrasthaṃ bhūta-grāmam acetasaḥ |
māṃ caivāntaḥ śarīrasthaṃ tān viddhyāsura niścayān ||17.6 ||
These foolish people, torture the material components of their bodies and Me also who dwell
within the body — know them to be non-divine in their faith.
So, Satavic person always follow the instrcutions which is given in Vedas/Upanisad.
This Sloka is also Siginficant here Chapter 16 Sloka 23
yaḥ śāstra vidhim utsṛjya vartate kāma kārataḥ |
na sa siddhim avāpnoti na sukhaṃ na parāṃ gatim || 23 ||
Whoever disregarding the ordinances of the Scriptures, acts under the influence of personal
desire, attains neither perfection nor happiness, nor the Supreme State.
Now, Truth is that other religons which are Man-Made, Hindusim is only religon which is come from the Lord Him-self where you can acheive the Supreme Truth.
India is KaramaBhumi as you know, because India is only place where you can do practice of Self-Realization and esepcially in Hindusim.
This Sloka is very Siginificant if some one is unsuccessful in practicing in Self-Realization here Chapter 6 Sloka 41,42,43
prāpya puṇya-kṛtāṃ lokān uṣitvā śāśvatīḥ samāḥ |
śucināṃ śrīmatāṃ gehe yoga-bhraṣṭo ‘bhijāyate || 41 ||
Having attained to the realms of the righteous and dwelt there for many long years, one who
has fallen from Yoga is born again in the house of the pure and prosperous.
athavā yoginām eva kule bhavati dhīmatām |
etaddhi durlabhataraṃ loke janma yadīdṛśam || 42 ||
Or one is born into a family of wise Yogis; but verily such a birth in this world is very
difficult to obtain.
tatra taṃ buddhi saṃyogaṃ labhate paurva-dehikam |
yatate ca tato bhūyaḥ saṃsiddhau kurunandana || 43 ||
There, O Arjuna, one regains the disposition of mind which one had acquired in the former
body, and from there one strives more than before for success in Yoga.
Now At-last This Sloka is Very Important Chapter 7 Sloka 19.
bahūnāṃ janmanām ante jñānavān māṃ prapadyate |
vāsudevaḥ sarvam iti sa mahātmā sudurlabhaḥ || 19 ||
At the end of many births, the enlightened one takes refuge in Me, realising that
‘Vasudeva is everything’— It is very hard to find such a great-person.
So, You have to understand that the ultimate achievement of innumerable good births — namely taking refuge in
Krishna. After passing through countless good births, one obtains the insight:— “I find
my ultimate joy in being a dependant (shesha) of Vasudeva. I am such that my essence,
existence and activities are completely under His control. He is superior to all others
because of His innumerable auspicious attributes.” After realising this, one takes refuge in
Krishna, ie., meditates on Him, with the understanding —
“Vasudeva alone is my highest goal and also the means for attaining it, and whatever other
desire remains in my mind, He alone, is all that for me too.”
It's Meaning is that you follow what-ever religons even Hindusim , but when you not understand that the every-thing is Lord Narayan then you have to take birth again and again whether you follow Islamic , Christian Buddhist or what-ever religions.
In this term this sloka is also very -siginficant Chapter 15.15
sarvasya cāhaṃ hṛdi sanniviṣṭo mattaḥ smṛtir jñānam apohanaṃ ca |
vedaiśca sarvair aham eva vedyo vedānta-kṛd vedavid eva cāham || 15 ||
And I am located in the hearts of all. From Me come memory, knowledge and their absence
also. Indeed, I alone am that which is to be known from all the Vedas. I bring about the fruition of
the rituals of Vedas; I alone am the knower of the Vedas.
Here Lord Krishna Declare that he is essense of the Vedas. and Lord Narayan(Vishnu) to be the supreme god as described by Vedas.
Even you learn the vedas by yourself you will come at this point that Lord Narayan is Supreme and his only final stage.But it is very difficult to learn Vedas by yourself, it will take 45 to 50 years or even long time to read the vedas and huge amount time to understanding the vedas maybe more than one birth.
So, the Solution for that Lord him-slef declare to the Arjuna because same dobut was in Arjuna's mind,it is impossible to learn and understand the vedas and its essence.
as Described in Bhagavad Gita very important chapter 4 Sloka 34
tad viddhi praṇipātena paripraśnena sevayā |
upadekṣayanti te jñānaṃ jñāninas-tattva-darśinaḥ || 34 ||
Just try to learn the truth by approaching a Spirtuial master (GURU). Inquire him submissively and render service unto him. The Self-realized souls can impart knowledge unto you becuase they have seen truth.
Acheive the Self-Realization you have to follow the Guru-Sisya Parampara
This Sloka is also very very Significant here:
śraddhāvāllabhate jñānaṃ tatparaḥ saṃyatendriyaḥ |
jñānaṃ labdhvā parāṃ śāntim acireṇādhigacchati || 4.39 ||
One who has faith, who is dedicated to serivce of Guru, and who has controlled the senses, attains
enlightenment. Having attained enlightenment, one obtains Supreme Peace.
Our Vedas have so much Information about the all things, even you can find in Kali-Puran or may be in bhavisya puran, everything is mentioned in that when Islam will born momhmad pagimbar will create the other religons and so, on everything .
But Remember one thing the truth, this all things are only for bewildering people, becasue Self-realization path is not so easy.
This Sloka perfect for it
manuṣyāṇāṃ sahasreṣu kaścid yatati siddhaye |
yatatām-api siddhānāṃ kaścin māṃ vetti tattvataḥ || 3 ||
Among thousands of men, perhaps one strives for perfection; even among those who strive for
perfection, one only may know Me; and among those who know Me, one alone perhaps, knows
Me in reality.
Among the thousand only one can know the supreme truth so, remember all this thing and follow the path of rightousness.
So, My advice to you if you follow the hindu dharma , read Bhagavad Gita follow the Guru ( only Authorized Spritual Master from the Vedas), because we already knew the importance of Guru, lord krishna himself declared that as I previously described in the Sloka.
But , At the end Lord krishna Himslef leave the all things on Arjuna whether he want to believe all thing or not. Chapter 18.63
iti te jñānam ākhyātaṃ guhyād guhyataraṃ mayā |
vimṛśyaitad aśeṣeṇa yathecchasi tathā kuru || 63 ||
Thus I have taught you that knowledge which is the greatest of all mysteries. Reflecting on it
thoroughly, do what you will.
So, Lord Krishna gave him Ulitmate truth but at-last Lord Krishna leave all thing on Arjuna, Becasue Lord never extort (steal) independence of Soul.
Here I can only give the knowledge form the Bhagavad Gita and through the scripture, accepting or not is upto you.
Hope This will Help.