While I was in a religious discussion with a Christian at work, I told him about Prophet Muhammad and explained to him he was prophisized by Isa too. The man rejected me and said to me plainly," it's not easy for someone to claim himself as prophet". I became frustrated and answered him in return and "even worse for someone to claim himself as God". I knew Isa didn't claim that but just to shut him up I said that. Do I have to ask Allah for mercy for saying such a thing just to answer him?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is obvious to you that Jesus (Peace and blessing be upon him), the messenger of Allah, neither claimed that he was God nor did he approve of it. And of course Allah announced his dispensation from the polytheism of the Christians. Allah Says (Interpretation of meaning): {When God said: “Jesus, son of Mary, have you told people, “take me and my mother as two gods instead of God (alone)”, he said, “Glory be to you? It is not my place to say what I have no right (Say): And (remember) when Allah will say (on the Day of Resurrection): “O 'Iesa (Jesus), son of Maryam (Mary)! Did you say unto men: 'Worship me and my mother as two gods besides Allâh?'” He will say: “Glory be to You! It was not for me to say what I had no right (to say). Had I said such a thing, You would surely have known it. You know what is in my inner-­self though I do not know what is in Yours, truly, You, only You, are the All­-Knower of all that is hidden and unseen. Never did I say to them aught except what You (Allah) did command me to say: ‘Worship Allah, my Lord and your Lord.’ And I was a witness over them while I dwelt amongst them, but when You took me up, You were the Watcher over them, and You are a Witness to all things. (This is a great admonition and warning to the Christians of the whole world)} [Quran 5: 116, 117]. No doubt, your saying "Even worse for someone to claim himself as God” is a grave mistake. You must turn to Allah and repent for it. You were right, when you said that Jesus gave glad tidings about The Prophet Muhammad. Though the Bible is distorted till this time, one can find some glad tidings that apply only to the Prophet Muhammad. For Example, see Deut. 17: 18 and 1: 33, John 7: 16. So forth.

Allah Knows best.