Is committing a sin in secret a major sin that leads to eternal damnation in Hellfire, or is it like shirk (polytheism) or kufr (disbelief)?

A sin whose perpetrator is not deemed a disbeliever by the Shari'ah does not become an act of disbelief if committed in secret. In fact, a sin committed in secret carries less guilt than one committed openly. A secret sin might be minor, expiated by avoiding major sins, or it might be a major sin requiring specific repentance. If the perpetrator does not repent from the major sin, they are not considered a disbeliever or a polytheist destined for eternal Hellfire; rather, they are under the Will of Allah.