A.T. Robertson Commentary Acts 13:25

"And as John was fulfilling his course, he said, What suppose ye that I am? I am not [he]. But behold, there cometh one after me the shoes of whose feet I am not worthy to unloose." — Acts 13:25 (ASV)

As John was fulfilling his course (ως επληρου Ιωανης τον δρομον). Imperfect active of πληροω, describing his vivid ministry without defining the precise period when John asked the question. Paul uses this word δρομος (course) of his own race (Acts 20:24; 2 Timothy 4:7).

What suppose ye that I am? (Τ εμε υπονοειτε ειναι?) Note τ (neuter), not τινα (masculine),

what not

who , character, not identity. It is indirect discourse (the infinitive εινα and the accusative of general reference).

Huponoeo (υπο, νοεω) is to think secretly, to suspect, to conjecture.

I am not he (ουκ ειμ εγω). These precise words are not given in the Gospels, but the idea is the same as the disclaimers by the Baptist in Joh 1:19-27 (cf. also Mt 3:11; Luke 3:16). Paul had a true grasp of the message of the Baptist. He uses the very form λυσα (first aorist active infinitive of λυω) found in Mr 1:7; Luke 3:16 and the word for shoes (υποδημα, singular) in all three. His quotation is remarkably true to the words in the Synoptic Gospels. How did Paul get hold of the words of the Baptist so clearly?