12 So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets.
12 The word "therefore" probably refers to the entire body of the sermon (5:17-7:12), for here there is a second reference to "the Law and the Prophets" (see comment on 5:17); Jesus stresses that he has taught about the true direction in which the OT law points, i.e., the Golden Rule. This rule sums up the Law and the Prophets (cf. Ro 13:9). In the context of fulfilling the Scriptures, it provides a handy summary of the righteousness to be displayed in the kingdom (cf. 5:20).
The verb translated "sums up" (lit., "is") might properly be translated "fulfills," as in Ac 2:16. In the deepest sense, therefore, the rule is the Law and the Prophets in the same way as the kingdom is the fulfillment of all that the Law and the Prophets foretold.
31 Do to others as you would have them do to you.
The so-called Golden Rule is not unique to Jesus, for it appears in various forms in the ancient world. Leviticus 19:18 says to “love your neighbor as yourself.” The philosopher Seneca wrote: “Let us show our generosity in the same manner that we would with to have it bestowed on us.” The negative version appears in Tobit 4:15 (“And what you hate, do not do to anyone,” nrsv) and is also attributed to Rabbi Hillel, a near contemporary of Jesus: “What is hateful to you, do not do to anyone else; that is the whole Law, all else is commentary.”
20 Whoever claims to love God yet hates a brother or sister is a liar. For whoever does not love their brother and sister, whom they have seen, cannot love God, whom they have not seen.
20 The confidence we have in knowing that God loves us delivers us from fear but not from responsible action. In fact, God's love for us and in us sets us free to love our brother and sister even as God loves them. To fail this test of love proves that one's claim to love God is a lie—just as the previous claims to have fellowship with God while walking in darkness (1:6), to know him while disobeying his commands (2:4), or to possess the Father while denying his Son were lies (2:22-23). John has a double sense in "liar." A liar does not speak the truth in that what he claims is false, and his actions show that he has divorced himself from the reality of God.
The second part of the verse is problematic. It most likely means that if one fails the test of loving a visible brother, such a one makes it certain that he or she does not love the invisible God; this proves that such a person has no true love at all.