He went up into the mountain (anebh ei to oro). Not "a" mountain as the Authorized Version has it. The Greek article is poorly handled in most English versions. We do not know what mountain it was. It was the one there where Jesus and the crowds were. "Delitzsch calls the Mount of Beatitudes the Sinai of the New Testament" (Vincent). He apparently went up to get in closer contact with the disciples, "seeing the multitudes." Luke ( Luke 6:12 ) says that he went out into the mountain to pray, Mark ( Mark 3:13 ) that he went up and called the twelve. All three purposes are true. Luke adds that after a whole night in prayer and after the choice of the twelve Jesus came down to a level place on the mountain and spoke to the multitudes from Judea to Phoenicia. The crowds are great in both Matthew and in Luke and include disciples and the other crowds. There is no real difficulty in considering the Sermon on the Mount in Matthew and the Sermon on the Plain in Luke as one and the same. See full discussion in my Harmony of the Gospels.