I'm by no means inclined to view Constantine as the ideal Christian or an Ideal Ruler. But I think the extent to which many people pin everything they don't like about mainstream Christianity on Constantine greatly oversimplifies things.
All the core doctrines of Catholicism had their seeds planted among the Church Fathers who came before Constantine. The reverence for the Greek Philosophy of Plato, Aristotle and Philo had already started. And to varying decrees so did the desire to reconcile Christianity with Rome.
Constantine did not formally make Christianity the state religion. In fact the Edict of Milan was actually an edict granting Freedom of Religion to all Religions.
"When you see that this has been granted to [Christians] by us, your Worship will know that we have also conceded to other religions the right of open and free observance of their worship for the sake of the peace of our times, that each one may have the free opportunity to worship as he pleases; this regulation is made that we may not seem to detract from any dignity of any religion."The common statement that Christianity didn't become the State Religion till Theodosius I is also perhaps an oversimplification. Christianity had certainly become a state sponsored religion before then. But only Gratian removed the state sponsored status of any element of the old Roman Religion.
It is with Theodosius I that Christianity as the state religion to the exclusion of other faiths began.
The original Nicene Creed of 325 AD contains nothing I object to (I just did a post defending the Homoousion doctrine). Nor anything I would expect to be objected to by most Protestants, Evangelicals, or other Christians who like to vilify Constantine and use Pre-Nicene and Post-Nicene to distinguish which Early Church Fathers they consider worth quoting.
Later the Creed was revised at the Council of Constantinople of 381 AD under Theodosius I.
"We believe in one God, the Father Almighty, Maker of all things visible and invisible.
And in one Lord Jesus Christ, the Son of God, begotten of the Father [the only-begotten; that is, of the essence of the Father, God of God,] Light of Light, very God of very God, begotten, not made, being of one substance with the Father; By whom all things were made [both in heaven and on earth]; Who for us men, and for our salvation, came down and was incarnate and was made man; He suffered, and the third day he rose again, ascended into heaven; From thence he shall come to judge the quick and the dead.
And in the Holy Ghost.
[But those who say: 'There was a time when he was not;' and 'He was not before he was made;' and 'He was made out of nothing,' or 'He is of another substance' or 'essence,' or 'The Son of God is created,' or 'changeable,' or 'alterable'— they are condemned by the holy catholic and apostolic Church.]"
"We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible.I don't mind the removing the Condemnation part. As a supporter of Eternal Security and now a Universalist, I have no desire to question the Faith of people I disagree with, even Arians.
And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds (æons), Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Ghost and of the Virgin Mary, and was made man; he was crucified for us under Pontius Pilate, and suffered, and was buried, and the third day he rose again, according to the Scriptures, and ascended into heaven, and sitteth on the right hand of the Father; from thence he shall come again, with glory, to judge the quick and the dead. ; whose kingdom shall have no end.
And in the Holy Ghost, the Lord and Giver of life, who proceedeth from the Father, who with the Father and the Son together is worshiped and glorified, who spake by the prophets.
In one holy catholic and apostolic Church; we acknowledge one baptism for the remission of sins; we look for the resurrection of the dead, and the life of the world to come. Amen."
It is mainly the last paragraph I object to, firmly establishing a terrestrially organized Church.
The other differences seem to be influenced by the earlier Old Roman Symbol and Rule of Faith creeds that goes back to the Second Century.
The Donations of Constantine were a forgery made by the Roman Catholic Church, though it gets used by Papal Antichrist theorists. That Constantine moved the Capital from Rome to Constantineople makes him if anything more a founder of the Eastern Orthodox Church then the Roman Church.
Theodosius I made Christianity the state religion and outlawed other faiths with the Edict of Thesselonica. I'm surprised more Historicists and other Papal Antichrist theorists haven't made a big deal out of that Edict being in the same city Paul directed his most Eschatologically concerned Epistles to.
However I feel Justinian's reign was equally important to formalizing what I consider corrupt mainstream Christianity. It was under his reign that Universalism was first condemned as a Heresy.
Update Aprils 7th 2017: Thing is while I agree with the Nicean Creed, other decisions the Council of Nicea made I'm not too fond of. Like their further elaborating on Ingaitus Church Governmental Scripture.
InspiringPhilosophy's video on Nicea is pretty good.