Satya Yuga is the first among the four yugas, where Dharma (religious principles) were followed to the perfection. Dharma can be found to be in perfect practice only in Rama Rajya and nowhere else. Naturally, we can deduce that the citizens of Rama Rajya are first class Vaishnavas, being devotees of Lord Ramachandra. Do we have any scriptural evidence supporting that every human was a Vaishnava in Krita Yuga?
In Srimad Bhagavatam, in Uddhava Gita of 11th Canto, Lord Krishna speaks these verses while describing varnashrama system.
SB 11.17.10 — In the beginning, in Satya-yuga, there is only one social class, called haṁsa, to which all human beings belong. In that age all people are unalloyed devotees of the Lord from birth, and thus learned scholars call this first age Kṛta-yuga, or the age in which all religious duties are perfectly fulfilled.
SB 11.17.11 — In Satya-yuga the undivided Veda is expressed by the syllable om, and I am the only object of mental activities. I become manifest as the four-legged bull of religion, and thus the inhabitants of Satya-yuga, fixed in austerity and free from all sins, worship Me as Lord Haṁsa.
This is generic trend in Satya yuga, there can be some exceptions always.
How can that be possible? For example, for any Dvija, it is mandatory to worship Goddess Gayatri who is nothing but an aspect of Adi Parashakti. So, any Dvija will be Shakta first before being something else. And this remains the same irrespective of which Yuga it is.
Moreover, Ramayana did not happen in Satya Yuga like you think. It occurred in Treta Yuga when lot of corruptions already crept in. People during Treta were not as pure as those who lived in Satya Yuga.
Also, in this case different Puranas might make different claims. For example, as shown in this answer, the Devi Bhagavatam says all people were primarily worshipers of Mahadevi (Parashakti) in the Satya Yuga.
Janamejaya said :-- “O Bhagavân! Describe particularly the details of the Yuga Dharma. I am now very desirous to hear which Dharma is for which Yuga?” 32-54. Vyâsa said :-- O King! I will now show to you by example the influence of the religion peculiar to each Yuga; hear it attentively. O King! The hearts even of saints are quite disturbed by the Yuga Dharma. See! Your father was a religious and high-souled monarch; still the wicked Kâlî defiled his mind and prompted him to do an act very insulting to a Brâhmana. Otherwise why would he, being a renowned prince amongst the Ksattriyas and a descendant of Yayâti, thus go and encircle a snake round the throat of an ascetic Brâhmin? Therefore, O King! All actions are being influenced by the Yuga Dharma. The Pundits, also recognise this. If you try your best to perform any religious act, even then the Yuga Dharma would prevail, yet you would be able to perform to a certain extent, a part of your intention. O King! In the Satya Yuga, the Brâhmins were versed in the Vedas, always devoted to worship the Highest Force, with an ardent desire to see the Devî; they were devoted to Gâyatrî with Pranava, devoted to the meditation of Gâyatrî, always reciting silently Gâyatrî, and the Mâyâvîja Mantram, the chief mantram. In every village, the Brâhmins were very eager to erect temples of the Devî Mahâ Mâyâ Ambikâ and were truthful, merciful and pure and devoted to their own respective works. The Ksattriyas, skilled in the science of the highest knowledge, were ever engaged in doing things ordained by the Vedas and were always intent in protecting well their subjects. The Vais’yas did their cultivation and trade and the S’ûdras always served the other three castes