{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 elattousthai, from
comparative elatt(936e), 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 ( Mt 11:2  Lu
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).