Our unity is not doctrinal. Our unity is spiritual (Eph. 4:3). We are not commanded to diligently preserve the unity of the faith, but to “diligently preserve” the unity of the Spirit. Do you have a doctrinal controversy with someone with whom you are united in Spirit? If God accepts both of you, which is the only way you can be united in Spirit, but you are doctrinally divided, then at least one of you is sinning. Maybe you are even intellectually correct on your doctrine, but you are sinning because division is a bad enough sin to eject you from the body of Christ (Tit. 3:10).
“Sound doctrine” has to do with obeying God, not figuring out theological ideas (Titus 2). In 1 Timothy 1:5 we read that the purpose of the commandment is love from a pure heart, a sincere faith, and a good conscience. Then Paul says that some, who have departed from that focus, have gone off into all sorts of wrong things. That is one of the greatest afflictions of the churches today.
If we could stay focused on laying aside sin and the weights that encumber us (Heb. 12), we would not have time for some of the doctrinal controversies we have.
There are a lot of opinions and rumors about Constantine, the Council of Nicea, and the events of the fourth century that changed Christianity to Christendom. Not only will you get the incredible story, with all its twists, plots, and intrigues, but you will find out how history is done and never wonder what is true again.