Matthew 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen.
When and where did they start teaching all nation.
Officially at Pentecost to Jews from all nations. Officially at Cornelius' house in Acts 10 to Gentiles.
“First, I thank my God through Jesus Christ for you all, that your faith is spoken of throughout the whole world.” Romans 1:8 (KJV 1900)
“For we have found this man a pestilent fellow, and a mover of sedition among all the Jews throughout the world, and a ringleader of the sect of the Nazarenes:” Acts 24:5 (KJV 1900)
“But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world.” Romans 10:18 (KJV 1900)
“If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;” Colossians 1:23 (KJV 1900)