How can one deal with satanic whispers and obsessions that cast doubt on forgiveness, the Divine Being, and prophethood, and how can one respond to the slanders of priests and Christians against Islam?

Your situation is peculiar; you are afflicted with obsessive thoughts and open the doors to doubts by arguing with disbelievers. How can you strengthen others when you yourself are shaken? You must busy yourself with truth, for if you do not, it will busy you with falsehood. Thoughts assail a person and cannot be cut off, but the strength of faith and intellect helps one accept what is good and repel what is evil. The self is like a constantly rotating millstone, it does not rest, and it must have something to grind. Either it grinds beneficial grains, or pebbles and sand. The most effective remedy is to busy yourself with what concerns you, not with what does not concern you, for thinking about what does not concern one is the root of all evil. Do not allow Satan to enter the house of your thoughts and will, for he will corrupt them for you. As for debating the Christian priest, we advise you not to engage in that, as it is a vast field of knowledge in which you are not specialized. If something from this priest's doubts has entered your heart, seek help from someone who has knowledge to resolve them. Know that this man is a liar and he knows that he is a liar. Allah Almighty says to His Prophet: (O Messenger! Let not those grieve you who hasten to disbelief from among those who say with their mouths, "We have believed," but their hearts have not believed; and from among the Jews, listeners to falsehood, listeners to another people who have not come to you. They distort the words from their [original] places, saying, "If you are given this, take it; but if you are not given it, then beware." And he whom Allah wills to put to trial - never will you possess for him [anything] against Allah. Those are the ones for whom Allah does not intend to purify their hearts. For them in this world is disgrace, and for them in the Hereafter is a great punishment.) Al-Ma'idah/41. These people who display faith but whose hearts do not believe, and the Jews who distort the words, are not to be paid attention to. And whoever Allah wills to put to trial, you will not possess anything for him, and for them in this world is disgrace and in the Hereafter a great punishment.