Loving One's Neighbour (cf. Luke 6:27 f., Matthew 5:32-36).Thou shalt love thy neighbour (i.e. fellow Israelite) is the precept of the Law (Leviticus 19:18); and hate thine enemy is a Rabbinic inference from, e.g. Deuteronomy 23:3-6, which found much support in apocalyptic writings (cf. pp. 623f.). As in the parable of the Good Samaritan, Jesus sweeps away all distinctions. The additions to Matthew 5:44 in AV are due to Luke 6:27 f.
Matthew 5:45. Sons are those who share their Father's character. If God were to give natural blessings like sun and rain to His friends and withhold them from His enemies, the natural world would be a chaos: in so far as His sons fall short of His nature the spiritual world is a chaos. Those who love only their friends miss the Divine reward (Luke 6:35), the attainment of the Father's character.
Matthew 5:46. What reward have ye? Justin Martyr has, Are you doing anything new? which is perhaps derived from an older text than ours. publicans: i.e. customs officers of inferior rank, the underlings of the publicani proper (p. 615); they exercised gross oppression and took money for an alien power, so that the Jews regarded them as outcasts, cf. p. 622.
Matthew 5:47 b. brethren may mean pious law-keeping Jews in view of early readings the godless, or tax-gatherers, in place of Gentiles.
Matthew 5:48. therefore sums up the teaching of Matthew 5:17-47; ye (my disciples) is emphatic; the future (shall be) has the force of a command. The comparison with the Divine character is far in advance of that noted in Leviticus 11:44; Leviticus 19:2 ff.