Which is correct, the sermon on the mount or the sermon on the plain? Some think Matthew 5:1, that locates Christ's sermon of the beatitudes on the mount, is contradicted by Luke 6:17 that supposedly locates the sermon on the plain.
his fame went throughout all Syria: and they brought unto him all sick
people…and those which were possessed with devils, and those which were lunatic,
and those that had the palsy; and he healed them.
5:1 And seeing the multitudes, he went up into a mountain: and when he was set, his
disciples came unto him:
5:2 And he opened his mouth, and taught them, saying…
5:3 Blessed are the poor in spirit for theirs is the kingdom...
6:12 …he went out into a mountain to pray, and continued all night…
6:13 And when it was day, he called unto him his disciples…
6:17 And he came down with them, and stood in the plain, and the company of his
disciples and a great multitude of people...came to hear him, and to be healed of their
6:18 And they that were vexed with unclean spirits: and they were healed.
6:19 And the whole multitude sought to touch him: for there went virtue out of him,
and healed them all.
6:20 And he lifted up his eyes on his disciples, and said, Blessed be ye poor: for
yours is the kingdom...
The sequence of events is revealed by cross-referencing. Luke tells of Christ spending a night in prayer on a mountain and then descending to the plain in the morning. He then speaks of Christ on the plain healing a multitude, then preaching the sermon. Luke doesn’t say Christ preached the sermon on the plain, only that He healed people on the plain before the sermon. And Matthew says He preached it on the mount, so the sermon was preached after Christ healed the multitude on the plain and ascended the mountain again. Luke doesn’t give the location of the sermon in 6:20 as Christ begins to preach after the healing on the plain, and Matthew doesn’t tell us the prior healing took place in the plain. We need only combine both gospels to see where both events took place.
It's evident from Matthew 4:24‑25 & 5:1 that Christ reacted to the multitude thronging Him for healing (on the plain) by ascending back up the mountain. On the steep terrain of the mountain, the multitude could not easily follow and throng about Him, permitting him to speak to His disciples of matters far more important than physical healing. This was much like His reaction to people who pressed upon Him to hear the Word of God in Luke 5:1‑3 when He entered into Simon's boat and thrust out a little from the land to preach to the people.
To summarize, Luke tells us Christ began to preach after He healed the people in the plain, and Matthew tells us that this later preaching did not begin until Christ had first ascended the mount a second time in reaction to the thronging by the healed and exhilarated multitudes.
Providence ordained cross-referencing for integrating all details of biblical events; the believer sees this, while the skeptic ignorantly scoffs.