1- It is not essential to take shahadah (say the testimony of faith) before witnesses. Islam is a matter that is between a person and his Lord. 2- No marriage contract is valid without a wali (guardian) for the wife, but no non-Muslim can act as a wali for a Muslim woman. For more, see the detailed answer.

Muslims believe that Jesus did not die on the cross and therefore there is no sacrifice, no salvation, and no trinity. Muslims do not believe in the original sin; rather they believe that no one can bear the sin of anyone else, or redeem him with his own self. Allah promises those who repent that their repentance will be accepted. The Messiah affirmed the importance of righteous deeds. He did not tell them of redemption by which they would be saved from the reckoning.