Theophylact of Ohrid

John did not convert all, but "many"; whereas the Lord enlightened all.

Commentary on Luke ↗

Ancient Greek Expositor

But what John's work is to be, and what he will do through the Holy Spirit, is shown as follows; And many of the children of Israel shall he turn, &c. Or else; The Jews were the parents of John and the Apostles; but, nevertheless, from pride and infidelity raged violently against the Gospel. Therefore, like dutiful children, John first, and the Apostles after him, declared to them the truth, winning them over to their own righteousness and wisdom. …

Catena Aurea by Aquinas ↗

Bede

And he will turn many of the children of Israel to the Lord their God, and he will go before him in the spirit and power of Elijah. When John, who bore witness to Christ, was baptizing people in his faith, he is said to have turned the children of Israel to the Lord their God; it is indeed clear that Christ is the Lord God of Israel. If, however, Christ is, or rather, since Christ is the Lord God of Israel, let the Arians cease from denying that Christ is the Lord God. …

On the Gospel of Luke ↗

Bonaventure

Thirdly, he commends him in doctrine with respect to the efficacy of usefulness and with respect to the eminence of authority. On account of the efficacy of usefulness he says: And he shall convert many of the children of Israel to the Lord their God. …

Commentary on Luke, Chapter 1 ↗