Did Later Christians Change What Early Christians Believed?

This important question results from the previous Q and A item, which summarized central teaching about Jesus Christ that has united most Christians since it was finalized by 5th Century ecumenical councils. It holds that the one true God exists in a Trinity of three persons, the Father, Son, and Holy Spirit, and that Jesus the Son has two natures, fully human yet fully divine. Myriad worshippers over centuries have professed each week that Jesus Christ is of one “being” or “substance” with God the Father.