Apostles' Doctrine

At Pentecost, Peter and the other apostles to the Church were filled with the Holy Spirit as they delivered God's message to those who were gathered and listening.

Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. Acts 2:41 KJV

God's Word does the work. Exposure to God's Word, delivered in the power of the Holy Spirit, can make the human soul aware that he is in desperate need of a savior. At Pentecost that Word was delivered orally, miraculously, by newly empowered believers speaking languages they had not learned. The New Testament Canon was yet to be written and translated, but God had other means of getting the people's attention and convicting their souls.

Isaiah 55:11-So shall my word be that goeth  forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it. 

Hebrews 4:12-For the word of God is quick, and powerful, and sharper than any two edged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.

Here’s the follow-through:

And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers. Acts 2:42 KJV

Peter’s message: identify yourselves with Jesus Christ. There is no reason to remain trapped in legalism. Salvation has been provided by God's grace.

Three thousand of those souls were added to the Church that day. They received baptism. They stayed with the apostles, to learn from them, fellowship with them, pray with them and share together their possessions and foodstuffs. Most of these people were pilgrims, remember, who had traveled there to celebrate the great feasts in Jerusalem. Now that they had entered a new life, their priority was to learn as much as they could about this life and the Author of it before going back to whatever place and occupations they came from. This was a time of great excitement, as they continued to meet and worship and learn and evangelize. More were saved each day, as this wave of Holy Spirit awakening continued. It would be hard to tear oneself away from such momentous happenings. One might rather sell his stuff and stay around for a while, soaking up the glory. And when they did go back to their own cities, they went back as born-again believers in Jesus Christ, having a magnificent testimony to share with the folks back home.

This is how the apostles' doctrine (truth about Jesus Christ) spread in the newborn Church.

Like fire.