Paul warned about women seeking after new teachings, and I don't think it was particularly different in pagan eras. Today, we bemoan the situation, as modern communications plus marketing messages to the widest possible consumer audiences means the planet being blanketed with spectacularly stupid things.
But naturally, people ensconced in our modern progressive narrative will suggest that there were evil state churches hither and yon. Well, the facts presented tend to be accurate, but the narrative never is. If the lords of Christendom had any property rights at all, one would have to- at least to some extent- admit that a 'state' church would be a much simpler affair than one existing now. Put simply, it would be an expression of the property owner's will as to what he wanted on his property.
Regardless, the Church of England, for instance, had little to no influence on the free market of religious ideas in England. Indeed, almost any sort of tripe can masquerade as Christian, since most folks tend to take a tribal view of it. The people may say they think this or that doctrine is important, but this is seldom true. Sentimentality that appears to be in accord with whatever tribe you want to get along with gets you a free pass, while worrying about doctrinal matters shall get you ostracized.
Within the realm, though, where everyone is safely CofE, or safely Roman (Catholic or otherwise), or safely Japanese (I am guessing) a wide range of religious practices are sold.
What are they selling?
Mystical answers to life's problems- different problems. There are many ways, but if you wanted a way to the supermarket and ended up at a donut shop instead, you would be a little disappointed.
Additionally, these mystical answers tend to have things in common. Better feels or practices to become better. People pay a lot for better feels, no matter how transitory they may be. In fact, I suspect most modern entertainment actually takes up a relatively low-level space in what should be understood as a religious realm.
But then there is that other side- the practices to improve, which tend to mesh well with Christian practices. Since the Trinity is a mystical answer to the human problem of not being able to achieve perfection, it was attractive to most who were attempting to improve themselves- all except those who were most vainglorious and unable to admit to themselves that no matter how good they were, they weren't perfect yet.