Do not associate any partner to Allah in worship
Associating partner/s to Allah in worship is the worst crime. It is the crime against our creator and so it is not pardonable by Allah. Any person who commits this crime becomes worthy to eternal punishment of fire. He or she is removed from the mercy of Allah, The creator.
It is narrated in one hadith that Jibreel (AS) came to Allah's messenger, Muhammed (SAW) and said that, any person from his followers who dies without associating any partner to Allah, will enter paradise.
So it extremely important for a believer, to worship Allah alone and not associate any partner to Allah in worship and obedience, Insha Allah.
And a person who is worshiping other created things or people instead of Allah, should give up, repent and believe in Allah and worship Him alone without associating any partner to Him.
May Allah guide us all and protect us from the eternal fire of hell. Ameen.