Mateo 18:1

Who then is greatest

(τις αρα μειζων εστιν). The αρα seems to point back to the tax-collection incident when Jesus had claimed exemption for them all as "sons" of the Father. But it was not a new dispute, for jealousy had been growing in their hearts. The wonderful words of Jesus to Peter on Mount Hermon (Mateo 16:17-19) had evidently made Peter feel a fresh sense of leadership on the basis of which he had dared even to rebuke Jesus for speaking of his death (Mateo 16:22).

And then Peter was one of the three (James and John also) taken with the Master up on the Mount of Transfiguration. Peter on that occasion had spoken up promptly. And just now the tax-collectors had singled out Peter as the one who seemed to represent the group. Mark (Marco 9:33) represents Jesus as asking them about their dispute on the way into the house, perhaps just after their question in Mateo 18:1.

Jesus had noticed the wrangling. It will break out again and again (Mateo 20:20-28; Lucas 22:24). Plainly the primacy of Peter was not yet admitted by the others. The use of the comparative μειζων (so ο μειζων in verse Mateo 18:4) rather than the superlative μεγιστος is quite in accord with the Koine idiom where the comparative is displacing the superlative (Robertson, Grammar, pp. 667ff.). But it is a sad discovery to find the disciples chiefly concerned about their own places (offices) in the political kingdom which they were expecting.