Albert Barnes Commentary Matthew 5:43

"Ye have heard that it was said, Thou shalt love thy neighbor, and hate thine enemy:" — Matthew 5:43 (ASV)

You have heard that it has been said, You shall love your neighbour, and hate your enemy. The command to love our neighbour was a law of God (Leviticus 19:18). That we must, therefore, hate our enemy, was an inference drawn from it by the Jews.

They supposed that if we loved the one, we must, of course, hate the other. They were total strangers to that great, peculiar law of religion, which requires us to love both.

A neighbour is literally one that lives near to us; then, one that is near to us by acts of kindness and friendship. This is its meaning here. See also (Luke 10:36).