Problem: Luke affirms that Jesus “stood on a level place” when He gave this famous sermon, but Matthew says “He went up on a mountain” to deliver it (Matt. 5:1). How can this discrepancy be resolved?
Solution: Granted that the two accounts are referring to the same event (see comments on Luke 6:20 below), they can be reconciled by noting that the mountain only refers to the general area where everyone was, while the level place denotes the particular spot from which Jesus spoke. It says “He stood on a level place.” It does not say all the people were seated in a level place. A level place from which to preach to a multitude on a mountain side would make a natural amphitheater.
“And he came down with them, and stood in the plain, and the company of his disciples, and a great multitude of people out of all Judaea and Jerusalem, and from the sea coast of Tyre and Sidon, which came to hear him, and to be healed of their diseases; (Luke 6:17)”