Person to person witness primarily. Before there was a NT, Jesus performed miracles before those of his day to prove to them that he was God in the flesh and could therefore also forgive sins and save souls.
He later empowered the apostles and some of the earliest church to perform miracles as well - and for the same reason - to display their authority and add to their credibility. He had ascended into heaven by that time.
Its easy to say "I can forgive your sins and save your soul" with no proof to that effect. But no one but God can perform miracles. So Jesus proved he was God through his miracles and then people believed he could forgive sins and save souls.
Mat 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:
Mat 28:20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
Rev 13:10 He that leadeth into captivity shall go into captivity: he that killeth with the sword must be killed with the sword. Here is the patience and the faith of the saints.