Revelation has a twofold aspect, esoteric and exoteric. The exoteric is intelligible enough to act as a vehicle for Grace, and this is the only real justification for adherence to any exoteric religion: to benefit from it as a means of Grace.
Since Grace is the only justification for adhering to religion, once one adheres to religion and enjoys the Grace which flows from it, he has no need of any other. For this reason he cannot be converted. The Muslim does not “need Christ” any more than the Christian “needs the Prophet.” Proofs either direction are futile because they have no point. Even if you could someone present a flawless case for Christianity to the Muslim, what does it matter to him? Again, one does not participate in religion for the truth of its claims–or perhaps we should say that this is only secondary since religion operates at the level of beliefs and not knowledge of the Truth. It operates at the level of Grace for the end of Salvation.
If some do convert, it is usually not under the best of circumstances and does more harm than good. Nonetheless, Grace can intervene for the improperly converted, but it does “retroactively.”
Anything that is formal is not unique. Only God can claim exclusive possession of a quality. This is summarized by the Gospel saying: “Only God is good.” The meaning is not that nothing else is good, since all that God created is good in its order. But only God is “only” anything. We may say that this or that teaching is good, but we may not say that only this or that teaching is good. Everything in formal manifestation is for that reason not in exclusive possession of a quality. If one expression of the good exists, it is, in a way, proof that many others exist. Likewise, the existence of one true religion is a kind of proof of the truth of others. “Only God is Truth.” To say “only Christianity is Truth” is blasphemy.