What is the appropriate response to Christians who argue using the verse: "And if you are in doubt about what We have revealed to you, then ask those who have been reading the Scripture before you. The truth has certainly come to you from your Lord, so never be among the doubters"?

The noble verse (So if you are in doubt about what We have revealed to you, then ask those who have been reading the Scripture before you) does not mean that the Prophet, peace be upon him, was in doubt; rather, it confirms his complete certainty in what was revealed to him. Suspending a ruling on a condition does not necessitate the occurrence or fulfillment of that condition, for example: "If you do not know me, then ask so-and-person," does not mean that the addressed person does not know the speaker. Thus, the verse indicates the negation of doubt concerning the Prophet, peace be upon him, and that the address is for whoever is in doubt, so that they may find certainty with the People of the Book who know the truth of what the Prophet brought. The intention is to establish proof against those who deny prophethoods, by clarifying that the People of the Book affirm what Muhammad, peace be upon him, brought.