2Corinthians 1:1
Galatians 1:13
Ephesians 1:22, and many more verses
Phillipians 3:6, 4:15
Colossians 1:18, and many more verses
1Thessalonians 1:1, 1Thessalonians is considered to be the first NT Book written in about 51 A.D.
2Thessalonians 1:1
1Timothy 3:5, 3:15, and 5:16
Titus 3:15, per added to the king James Version
Philemon 1:2
Hebrews 2:12 and 12:23
James 5:14
1Peter 5:13
3John 1:6, 1:9-10
Revelation 2:1, plus six more in the same book.
Since all of these books mention the word ‘church’, the church had to have been in existence already for years before they were written.
If that is true, then how was the teaching accomplished without a NT of Jesus Christ?
Simply by the same way that Jesus Christ taught, orally. Oral teaching is Tradition (capital T) is Apostolic oral teaching handed down to their successors and to others.
The Bible is oral Tradition written down.
There was no such thing as Sola Scripture even then.
There was not any NT Scripture for the first twenty years of the existence of the church.