God is sovereign, and He chooses to use man to spread the gospel. Could He have done it another way? Of course, He's God!
And, whether you accept it or not the word of God expresses that everyone living in the NT must repent, and be water baptized in the name of Jesus for remission of sin and receive the Holy Ghost in order to gain entrance into the kingdom. (John 3:3-5, Luke 24:47, Acts 2:36-42, 8:12-18, 9:17-18, 10:43:49, 19:1:10, 22:16...) Those who believe what God says, and obey what was initially expressed by the Apostle Peter, come to know the truth, and that truth makes them free. It is only after the act of faith that one's spiritual eyes become open.
"And being made perfect, he became the author of eternal salvation unto all them that OBEY him;" Heb. 5:9
"Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;
And ye shall know the truth, and the truth shall make you free." John 8:31-32
"And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth.
Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
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." Matt. 28:18-21
The following scripture reveals so much more than people realize. Those who get water baptized without ever believing what God says on the subject come out of the water no different than when they entered. However, those who believe and submit to baptism come out of the water a new creation in Jesus. (2 Cor. 5:17) Why? Because God said so!
"And he said unto them, Go ye into all the world, and preach the gospel to every creature.
He that believeth and is baptized shall be saved; but he that believeth not shall be damned." Mark 16:15-16