The Church has never ceased to be. There is unbroken continuity, historically and ontologically, between the one holy Church of the New Testament and the present Church. One characteristic of that undying Church is that it is fully the Body of Christ.
“And He put all things in subjection under His feet, and gave Him as head over all things to the church, which is His body, the fullness of Him who fills all in all.” (Ephesians 1:22-23 NASB).
Another is that the Church is apostolic. This includes, in part, that the Church hold to and practice all that the Apostles taught and practiced.
“But we should always give thanks to God for you, brethren beloved by the Lord, because God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth. It was for this He called you through our gospel, that you may gain the glory of our Lord Jesus Christ. So then, brethren, stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us.” (2 Thessalonians 2:13-15 NASB)
“Now we command you, brethren, in the name of our Lord Jesus Christ, that you keep away from every brother who leads an unruly life and not according to the tradition which you received from us.” (2 Thessalonians 3:6 NASB)
“They were continually devoting themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer.” (Acts 2:42 NASB)
And the Church is “the household of God, which is the church of the living God, the pillar and support of the truth.” (1 Timothy 3:15 NASB)
So, in a very truncated form, I am arguing that departure from the Apostles’ teaching, practice, and tradition, is tantamount to a departure from the Church, in whom the fulness of Christ dwells, which is the Body of Christ. Or, as the Church Fathers call it, a theandric (lit., “divine-human”) entity.