Documentation proving Schaff's presupposition on historical corruption of churches everywhere post 300 a.d.

Does anyone know where I can find any documentation backing schaff's assertion that most evangelicals accept in regards to entrance of false doctrines in the church starting post 300 a.d. please? When I look at the orthodox, catholic, syrian, coptic and other churches that were not in synch geographically or even all under the Christian and nonChristian Roman emperor's attacks and protections, I seem to find similar beliefs in regards to what happened to Mary post her death and the universal church's acceptance of what is true about her role in Heaven. This is not the same as the post 1850 version of the catholic bishop's (Pope) dogmatic declarations, but the concept of theotokos, Mary's womb being the sanctified special holder of God the divine human being, therefore specially settng her apart, and her appearances to humble sincere believers post her saintly presence as the one worshipping Christ as LORD and Savior forever as well as given the blessing of being by Christ's right side in heaven, seems to be found in all churches everywhere, even those with some conflicts with one another.

The universal councils of the faithful who were deeply concerned with protecting all TRUTH in regards to what was truly divine and TRUE also emphasized our LORD's divinity, Mary as "Theotokos" for those true believers who worshipped Christ as LORD GOD in the flesh. Also, prayers during Church along with Mary and other saints, during Revelations like incense filled Church services, seem to have been part of almost all churches everywhere. Wonder if anyone has found documentation showing the Christian emperor overruling all Church pastors to force down false doctrines to churches in the future through his interventions? All I have found thus far, is a Christian trusted source in Eusebius' writings describing Christ as LORD of all old and new testament as well as a humble emperor constantine bowing to the Power of God present in the faithful Bishops who had kept the faith at the cost of limbs, eyes and much torture during their lives; but I can not seem to find accounts of his desire to corrupt the church or the church's willingness to receive anything false, but rather, even the top bishops, like Arius, seem to be punched out physically by deacons when they attempted to change any doctrines that were not in accordance to the united testimony of the church since its beginning.

I know much of us have been taught and teach that Roman Christians have perverted some of these truths in regards to the presence of saints and Mary, the Mother of God, the Theotokos, during Church services, but I am having an impossible time finding historical reality to this commonly accepted modern post 1500 belief common among us believers in the west now. Appreciate all the help you can provide me with this area.