The book of the prophet Isaiah was handed to him. He opened the book, and found the place where it was written,
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,"