Constantine was shall we say a "token Christian" and didn't get baptized until he was on his death bed believing that that would allow him to sin all he wanted before playing his "get out of hell free card".

He stopped the persecution of Christians, for which I am thankful, but by getting the government involved he made Christian leadership desirable for people seeking personal wealth, power, and glory.

This plagues the Roman church to this very day.

But despite this, Constantine knew nothing about what books were authentic and what were not and deferred to what the majority of Christian leaders at the time had been using for over 200 years.

So, while Constantine had horrible impact on how the Roman / Eastern church departed from truth for the rest of history, he did not affect what is Biblical Christianity (Protestantism) because that depends only on what the original Greek of the original writers with demonstrable apostolic authority.

I have looked into this in detail while teaching a Bible study series on where the elements of the Apostle's creed came from. It's all in the Bible and the creed faithfully summarizes it.