Jesus answered him, "Get behind me Satan! For it is written, 'You shall worship the Lord your God, and him only shall you serve.'"
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,"