Unforgivable sin of associating anything [partners, helpers, other gods] with Allah
The Qur'an accuses Christians for their belief that Jesus is Lord and God, calling them unbelievers (kafiroon) and idolaters (mushrikoon), i.e. people who are committing shirk:
And the Jews say, ‘Ezra is the son of ALLAH,’ and the Christians say, ‘the Messiah is the son of ALLAH;’ that is what they say with their mouths. They only imitate the saying of those who disbelieved (kafaroo) before them. ALLAH's curse be on them! How they are turned away. They have taken their priest and their monks for lords besides ALLAH. And so have they taken the Messiah, son of Mary. And they were not commanded but to worship the One God. There is no God but HE. Holy is HE far above what they associate (yushrikoona) with Him! They seek to extinguish the light of ALLAH with their mouths; but ALLAH refuses but to perfect HIS light, though the disbelievers (al-kafiroona) may resent it. HE it is Who has sent HIS Messenger with guidance and the religion of truth, that HE may make it prevail over every other religion, even though the idolaters (al-mushrikoona) may resent it. S. 9:30-33 Sher Ali