“Let love be genuine.”
Familiar verses describe the positive qualities of Christian love — that it is patient, kind, rejoices with the truth, is full of hope, and so on. Other qualities of Christian love are expressed by the New Testament writers as the absence of something bad — not arrogant, not rude, not selfishly insistent, not resentful.
Genuineness is a positive quality, but the word underlying Romans 12:9 is actually one of these Greek negations. We might translate it “not-fake”. Reflecting this, other translations go with “unfeigned”, “without hypocrisy” and “without dissimulation”, the last of which may be a little too archaic to be much use.
It reminds us that loving in truth demands we avoid insincerity.