About this passageThe last of a series in the Sermon on the Mount in which Jesus quotes a received teaching and then extends it — "you have heard that it was said … but I tell you". The argument turns on the weather: sun and rain fall on everyone regardless of merit, and the listener is told to love on the same terms. The closing word translated "perfect" carries the sense of complete or brought to its full end, rather than flawless.
"You have heard that it was said, 'You shall love your neighbor and hate your enemy.' But I tell you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who mistreat you and persecute you, that you may be children of your Father who is in heaven. For he makes his sun to rise on the evil and the good, and sends rain on the just and the unjust.
For if you love those who love you, what reward do you have? Don't even the tax collectors do the same? If you only greet your friends, what more do you do than others? Don't even the tax collectors do the same? Therefore you shall be perfect, just as your Father in heaven is perfect."
In plainer words
You have been told to look after your own and hate the people against you. I am telling you to love the people against you — wish them well, and mean it. That is what it looks like to belong to God, who sends sunshine and rain on decent people and awful people alike. Loving people who love you back is nothing special; anyone does that. Being greeted by your friends is nothing special either. Go all the way, the way God does.
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.