Other than rejecting the only begotten Son of God, I suspect another surefire way of guaranteeing your reservation in Hell would be calling God a transgender, too.
Deuteronomy 22:5 The woman shall not wear that which pertaineth unto a man, neither shall a man put on a woman’s garment: for all that do so are abomination unto the LORD thy God.
No single sin, or any finite number of sins, will do this (although some self-centered Christian “callings” will claim that they do) — but the unique sin of refusing God as Savior. Make any other mistake about Him, but do not make this mistake. Because if accepted as Savior, God will gladly correct the other mistakes — but that mistake is an everlasting mistake.