Vigorous missionary work is one of the characteristics of the true Church.
He spent most of the time training them. The Sermon on the Mount was not a public address to a multitude, but a missionary training and preparation meeting for the Twelve and a few others. This is made clear by Matthew 5, 6, 7 in the JST.After ordaining and preparing the Twelve and the Seventy, He sent them on short missions, in pairs, throughout the villages and cities of the Holy Land to give them experience. (Matt. 10:1-5; Luke 9:2,6,10; 10:1-12, 17,18.) When they returned from their missions, Jesus took them to a secluded area and heard their reports and counseled them. (Luke 9:10.)
Jesus had a timetable. When He sent the Twelve on their first missions He told them to preach only to Israelites - not to Samaritans or to Gentiles. (Matt. 10:5-7.) However, after His resurrection He instructed the Twelve to go to all nations in the world. (Matt. 28:16-20; Acts 1:5-8.) The book of Acts is an account of how the Church did missionary work after the ascension of Christ into heaven. Acts 1-7 deal with the Twelve teaching the Jews in and around Jerusalem; Acts 8 recounts Peter and John extending the gospel to the Samaritans; and Acts 10-28 tells of missionary labor among the Gentiles. This is precisely the sequence Jesus directed in Acts 1:8.
Missionary work among the Samaritans did not produce a culture shock because they already had the Law of Moses, much the same as the Jews did. However, missionary work among Gentiles required a major cultural adjustment in the church. It was proper that it was introduced by Peter, the president, holding the keys of the priesthood. Serious missionary work directly with the Gentiles began with Cornelius as told in Acts chapter 10 and 11. Later, an important landmark conference in Jerusalem (Acts 15) about 49 or 50 A.D. determined that the gospel of Christ should be taken directly to the Gentiles, without the Law of Moses. This was a major decision affecting missionary procedure and was presided over by the apostles, with Peter at the head.
Paul's missionary methods are clearly shown in Acts. In each city, he generally started at the synagogue. (Acts 13:5,14; 17:1-3.) He preached from the scriptures and testified that Jesus is the Christ. His main intent was to contact the Gentiles. He often did this by going to the synagogue to find Gentile persons (Greeks) who had already joined the Jewish religion. Such are called "proselytes." (See LDS Bible Dictionary, p. 754.) These he would convert to Christ. These then would bring their relatives and friends to be converted. This was a referral system much like what we have in the Church today. (See Acts 13:42-49.)