Idolatry - the worship of someone or something other than Yahuwah as though it were God. The first of the biblical Ten Commandments prohibits idolatry: "You shall have no other gods before me." We are committing idolatry if we worship anyone but the true God of the bible.
Hebrews 6:4-6 appears to describe an unpardonable sin, someone who is a true believer making a final and irrevocable rejection of the gospel.