"The Spirit of the Lord is on me, Because he has anointed me to preach good news to the poor. He has sent me to heal the brokenhearted, To proclaim release to the captives, Recovering of sight to the blind, To deliver those who are crushed,
And in the Greek, the if is in the subjunctive, rather than in the indicative case. And the if in the subjunctive is not a question, but is a declaration, "Since thou art the Son of God". He isn't really questioning the fact if Jesus is the Son of God or not, but he is in reality saying, "Since thou art the Son of God,"