When you think about it, early Christians were pretty insignificant and barely even mentioned in archaeological finds. It's pretty clear they were looked upon as a nuisance and not very well liked making their popularity pretty much nonexistent until Constantine forced Christianity upon people and later by the Catholic Church who refined the 'flavor' of the religion to what it is today.
Helluva way to become 'popular'.
Respectfully, your historiography -
or as it seems, major lack thereof - is dreadfully misinformed. Here are the two respects in which you err, rather significantly.
1. Constantine did not
'force' Christianity upon the Empire. He was actually very tolerant of Paganism (even continuing to act as
Pontifex Maximus for the traditional Roman state cult and using Sol Invictus, the Sun God, as the emblem of his regime). He only got himself baptised into the church on his deathbed, because he so badly wanted to retain the trust of both polytheists and Christians in his multi-religious state, despite his clear preference for Christian doctrine and personal belief in it.
Religious tolerance was Constantine's official imperial policy:
"...Each one may have the free opportunity to worship as he pleases; this regulation is made we that we may not seem to detract from any dignity or any religion...It is one thing acting with free will to enter into contest for immortality, another to compel others to do so by force through the fear of punishment. No one should greatly trouble another, rather, everyone should follow what his soul prefers..."
- Constantine the Great (c.272 - 337) (Elizabeth DePalma Digeser, in The making of a Christian Empire: Lactantius & Rome)
Some scholarship on this:
"...
In principle he (Constantine) treated religion as a matter of choice and conscience, an arena free of state meddling...Liberis mentibus — "With Free minds" — all are to worship their Gods. It is a remarkable policy, an unexpected one, since it would have been natural for a ruler after his conversion to Christianity to shift all the previous relations. …
Most of the apologists who defended the Church in the early centuries advocated freedom of religion...the latin rhetor Lactantius developed a theological arguement for religious freedom. Lactantius was close enough to Constantine later to serve as tutor to the emperor's sons, and his influence is evident in many ways in Constantine's own writings. …He (Lactantius) asked those who believed in compulsion of religion: "What good can you do, then, if you defile the body but cannot break the will?" It is a surprisingly modern statement, arguing, that religious freedom is the "first freedom", rooted in the very nature of religious life as an exercise of free will...Under Constantine's policy of concord, the Church was flooded with new converts, not through coercion but by force of Imperial example...Eventually, Christian Emperors abandoned Constantinian religious policy...
Constantine favoured the Church but gave serious attention to protecting the rights of non-Christians. One cannot help but muse how European history would have been different if Christians had had the patience to let Constantine's original settlement alone..."
- Peter J. Leithart, in Defending Constantine : The Twilight of an Empire and the Dawn of Christendom (2010)
He likely converted because it was already becoming a significant movement in the Empire, and calculated that he needed to grasp some of its populist appeal amongst the masses to establish his grip on power in the aftermath of the decades long civil war that had ruptured the Roman world since the crisis of the third century, as well as thinking that monotheism would help unify the empire.
2. The rise of Christianity was a hotly disputed phenomenon in the Roman Empire. It was classified as a depraved superstition — the Latin term
superstitio, and therefore received much scrutiny and was the subject of intense paranoia. By the end of the
first century,
Christianity had already spread to
Rome and major cities in
Armenia,
Greece and
Syria and already becoming the dominant religion in many urban centres by the late 3rd century. Undoubtedly, Constantine's conversion helped it grow exponentially - more from patronage and desire to imitate the emperor, than enforcement - but it was hardly negligible before the Edict of Milan in 312.
As one of the foremost scholars on Early Christianity, Professor Larry Hurtado, explains:
"
Christianity did not become successful through Constantine giving it imperial approval. Instead, Constantine adopted Christianity likely because it had already become so successful despite earlier efforts to destroy the movement...
Christianity was certainly not part of the establishment…[and] was considered…a dangerous development that challenged what were then accepted notions of religion, piety, identity, and behaviour...
In the eyes of many of that time, early Christianity was odd, bizarre, in some ways even dangerous. For one thing, it did not fit what “religion” was for people then. Indicative of this, Roman-era critics designated it as a perverse “superstition.” Yet the very features of early Christianity that made it odd and objectionable in the ancient Roman setting have become now unquestioned assumptions about religion in much of the modern world.”
― Larry W. Hurtado,
Destroyer of the gods: Early Christian Distinctiveness in the Roman World
And another, Bart Ehrman in his 2018 book,
The Triumph of Christianity: How a Forbidden Religion Swept the World:
Constantine’s conversion was not the single decisive turning point in the spread and success of the Christian religion, the one moment that changed all history and made the Christian conquest a success. At the rate it was growing at the time, Christianity may well have succeeded otherwise. If Constantine had not converted, possibly a later emperor would have done so—say, one of his sons.
We have so many references to it in extant pagan literature of the time, including Pliny the Younger's Letter to the Emperor Trajan about how to handle arrested female Christian deacons in 112 A.D,.Tacitus's
Annals, 15.44 in 113 A.D. which describe the Neronian persecution of Christianity, the
Meditations (161 to 180 A.D) of Emperor Marcus Aurelius which refer to Christians and the peculiar aspects of Christian doctrine multiple times (
I.6, III.6, VII.68, VIII.48.51, and XI.3.), the Roman physician Galen (129 AD – c. 216) in
De pulsuum differentiis, ii & iii and the satirist Lucian of Samosata (c. 125 AD – 180 AD) who wrote about Christian beliefs extensively in his
Death of Peregrine, even noting what was by then common knowledge among the Roman public, "
The Christians, you know, worship a man to this day,–the distinguished personage who introduced their novel rites, and was crucified on that account", he goes on to say:
The Death of Peregrine | De Morte Peregrini | The Lucian of Samosata Project
In some of the Asiatic cities, too, the Christian communities put themselves to the expense of sending deputations, with offers of sympathy, assistance, and legal advice. The activity of these people, in dealing with any matter that affects their community, is something extraordinary; they spare no trouble, no expense...
These misguided creatures start with the general conviction that they are immortal for all time, which explains the contempt of death and voluntary self-devotion which are so common among them; and then it was impressed on them by their original lawgiver that they are all brothers, from the moment that they are converted, and deny the gods of Greece, and worship the crucified sage, and live after his laws.
All this they take quite on trust, with the result that they despise all worldly goods alike, regarding them merely as common property. Now an adroit, unscrupulous fellow, who has seen the world, has only to get among these simple souls, and his fortune is pretty soon made; he plays with them.
Christian beliefs were variously scorned, ridiculed, feared and subject to intense scrutiny. Everyone had a 'position' on what Lucian referred to as, "
their queer creed".
