The "multitudes"or "crowds"consisted of the people Matthew just mentioned in 4:23-25. They comprised a larger group than the "disciples."
The disciples were not just the Twelve but many others who followed Jesus and sought to learn from Him. Essentially "disciple"means learner. They did not all continue to follow Him (John 6:66). Not all of them were genuine believers, Judas Iscariot being the notable example. The term "disciples"in the Gospels is a large one that includes all who chose to follow Jesus for some time anyway (Luke 6:17). We should not equate "believer"in the New Testament sense with "disciple"in the Gospels.231
"To say that every Christian is a disciple' seems to contradict the teaching of the New Testament. In fact, one could be a disciple and not be a Christian at all! John describes men who were disciples first and who then placed their faith in Christ (Jn. 2:11). . . . This alone alerts us to the fact that Jesus did not always equate being a disciple' with being a Christian."232
Customarily rabbis (teachers) sat down to instruct their disciples (cf. 13:2; 23:2; 24:3; Luke 4:20).233This posture implied Jesus' authority.234The exact location of the mountain Matthew referred to is unknown, though probably it was in Galilee near the Sea of Galilee and perhaps near Capernaum.
"There is probably a deliberate attempt on the evangelist's part to liken Jesus to Moses, especially insofar as he is about to present the definitive interpretation of Torah, just as Moses, according to the Pharisees, had given the interpretation of Torah on Sinai to be handed on orally."235
The phrase "opening His mouth He began to teach them"(v. 2; NASB) or "He began to teach them"(NIV) is a New Testament idiom (cf. 13:35; Acts 8:35; 10:34; 18:14). It has Old Testament roots (Job 3:1; 33:2; Dan. 10:16). It introduces an important utterance wherever it occurs.
There is some difference between preaching (Gr. kerysso; 4:17) and teaching (Gr. didasko; 5:2) as the Gospel writers used these terms (cf. Acts 28:23, 31). Generally preaching involved a wider audience and teaching a narrower more committed one, in this case the disciples.
