A Biblical Example of why Discipleship is so important.
(Acts 19:8-10) "And he went into the synagogue and spoke boldly for three months, reasoning and persuading concerning the things of the kingdom of God. But when some were hardened and did not believe, but spoke evil of the Way before the multitude, he departed from them and withdrew the disciples, reasoning daily in the school of Tyrannus. And this continued for two years, so that all who dwelt in Asia heard the word of the Lord Jesus, both Jews and Greeks."
Paul had an extended time of preaching in the synagogue, but eventually, the influence of the Jews who rejected the message drove him out. He then resumed his teaching in the hall of a Gentile teacher named Tyrannus.
Paul did this daily, meaning every day. Considering his extended time in Ephesus, this meant many hundreds of hours of teaching. It is no wonder that the work in Ephesus was so broad and effective.
Paul carried this on for two years, and his effective teaching equipped believers, who got the word of God out to all who dwelt in Asia.
By himself, there was no way that Paul could reach this entire region. But he could equip Christians to do the work of the ministry, just as he described in Ephesians 4:11-12.
This is why Discipleship is so important. We grow infant Christians into mature Christians who can go and teach others. This cycle has never stopped since Jesus called the first two fishermen on the shore of Galilee to follow Him.