Even when Satan says truth, even when he quotes Scripture, he puts a perverse twist to it. How did our Lord fight Satan? With truth! That is how one defeats Satan: being confident that Jesus Christ has already secured the victory and that God has put a hedge around us so that we will not fall into a situation confronting Satan that is beyond us, and being absolutely reliant upon the truth of God! Even though we may not be able to see how it is worked out, even though we may feel that following the truth of God is going to require a considerable sacrifice on our part, we have the example of Jesus Himself fighting Satan by relying upon the truth of God. He trusted what God said.
One might wonder why Satan used "if." He did not use "if" to get Jesus to doubt His Sonship. Jesus knew who He was. Rather, he was trying to get Him to reflect on the meaning of "if." Satan seems to be saying, "Surely, if You are the Son of God, You have the right to expect Your needs at the moment to be satisfied."
Jesus did not fall for it. As hungry as He was, He knew it was a trap. He knew He did not have to be concerned about supplying His material needs because God would do it for Him. Did He not later say, "If God so feeds the birds of the field"?
This was a temptation for Christ to use His Sonship in a way other than its God-ordained purpose. What is the God-ordained purpose of our calling? "Seek first the Kingdom of God and His righteousness, and all these things will be added to you" (Matthew 6:33). That is the truth of God. God will supply what we need. So Jesus' answer was, "Thank you, but I'll just wait for God to supply My need."