Must (dei). It has to be (see 3:14 ). He is to go on growing (present active infinitive auxanein) while I go on decreasing (present passive infinitive elattousqai, from comparative elattwn, less). These are the last words that we have from John till the despondent message from the dungeon in Machaerus whether Jesus is after all the Messiah ( Matthew 11:2 ; Luke 7:19 ). He went on to imprisonment, suspense, martyrdom, while Jesus grew in popular favour till he had his via dolorosa. "These last words of St. John are the fulness of religious sacrifice and fitly close his work" (Westcott).