Basically, I want to know about Christians before the Nician Creed, and what their relationship with magic might have been (since pagan religions used it quite a bit). Are there any records of Christian sects like the Gnostics using what might be considered 'occult' practices - such as ceremonies, prayers, spells, or practices that were later condemned after Catholicism took root? Any sources, books, or articles would be appreciated.
Keep in mind that while there was no true settled canon of "New Testament" Scriptures, even at the time of Nicaea there hadn't been a real settled NT scripture. Saint Athanasius, Patriarch of Alexandria was one of the first the first to suggest what we use as our canon of the New Testament, and this was in his pastoral letter for Pascha (he was a Deacon at the time of Nicaea).
However, the Christians had the Septuagint (Greek old testament) and there are some pretty clear condemnations of magic/sorcery. Also, the books we know in the NT were read during the services of the Church, while not later canonized, the letters of Hermas, and a few other epistles were read. So, weekly, or daily the people were being educated about what the faith taught.
You can also look at the life of Saints Cyprian and Justina. To summarize,v Saint Cyprian was a magic user who many people would come to, so he would cast spells for them. At one point someone wanted Saint Justina to fall in love with them and came to Cyprian. So, using magic/his relationship with demons sent them to change her. Justina used the sign of the cross and the demons could have no affect. They came back and told him, and he knew that there was real power in Christ. So he fled to the nearest Church, and begged to be let in. The priest was skeptical and told him to wait, to which Cyprian said "No, I know whats out there, and I want what this faith has to offer." He became a Christian, and eventually was made a bishop. This was in the early 300's
I've also read somewhere (I think it was in the books by Kallistos Ware) that in the very first centuries there were pagan worshipers who tried to take particles of the Eucharist and use them in their spells (since it was the body and blood of God).
So, well before Nicaea there was no room for magic within Christianity.
It's also worth noting that Nicaea did not deal with the canon of Scriptures. It was called in response to Arianism. They also discussed and wrote out canons (rules) about the reception of people who had denounced the faith under persecution, the requirements of clergy, and a few other things.