According to wikipedia:
In Islam, shirk is the sin of idolatry or polytheism, i.e. the deification or worship of anyone or anything other than the singular God, or more literally the establishment of "partners" placed beside God. It is the vice that is opposed to the virtue of tawhid (monotheism). Within Islam, shirk is an unforgivable crime; God may forgive any sin except for committing shirk ... In the context of the Qur'an, the particular sense of "sharing as an equal partner" is usually understood, so that polytheism is "attributing a partner to Allah". ...
My understanding is that shirk is ascribing partners or equals with Allah (swt). We know that many mushrikun repented and converted to Islam, for example with the conquest of Mecca. We also know that Prophet Muhammad (saaws) pleaded with his polytheistic uncle, Abu Talib, on his death bed saying, "O uncle! Say: None has the right to be worshiped but Allah, a sentence with which I shall be a witness for you before Allah." Even though as a Christian I once worshiped Jesus as God, I have repented and asked Allah (swt) to forgive me.
Christians are shown to commit the sin of shirk as shown by John 3:16 and the Quran 19:88-92 They say: "((Allah)) Most Gracious has begotten a son!" Indeed ye have put forth a thing most monstrous! At it the skies are ready to burst, the earth to split asunder, and the mountains to fall down in utter ruin, that they should invoke a son for ((Allah)) Most Gracious. For it is not consonant with the majesty of ((Allah)) Most Gracious that He should beget a son.
My understanding is that it would not be a good thing to die as a Christian, worshiping Jesus as God. The concepts of Trinity and Tawhid are definitive in distinguishing the 2 religions.