I'm a Quranist. I only accept the Quran as word of God and religious doctrine. Most Muslims (Sunnis/Shias) would consider me an apostate because I don't follow their man made Hadith books. Ok so here's the deal from a Quranic point of view... Our ONLY purpose for existence is to worship God alone. No partners, no sons or daughters, no statues, no idols, no assistant gods, no saints, no ancestors, no graves of dead relatives, no spirits... Just God alone. As for one of the previous posts, murders, rapists, etc will have their punishment. These are sins according to Quran. But no sin compares to worshipping anything other than God alone. That is the greatest sin as we are denying the purpose of our creation. And if you die an unbeliever (polytheist), the punishment is eternal hell. If you repent before death, God is most forgiving. I find this totally fair. To worship God, you must let go of ego. At least in front of our creator. We would not exist were it not for him. He gave us free will. So if you want to pray to a monkey penis, that was your choice. Deal with the consequences. Must deny God out of pride and ego.
As for other religions, polytheist religions. Yes, Quran condemns the worship of anything other than God alone. And warns them of the day of judgment. But punishment for polytheists belongs to God alone. They'll get theirs on the day of judgment. We humans have no right to punish them here on Earth for their lack of faith. That's between each individual and God. All the pig faced scholars who are calling for the murder of apostates, idol worshipers, adulterers and homosexuals... Are following man made books (Hadith) or just making up baseless fatwas. God gives them no right to change or add to his laws. These ridiculous laws you will not find in the Quran. According to the Quran, we are only allowed to fight, kill in cases of self defense, oppression, if you are being kicked out of your homes or are not being allowed to practice your religion. If the non-believers are not doing any of the following, we are told to be equitable with them. Don't believe the pig faced scholars. They don't follow Quran.
[60:8] God does not prohibit you from those who have not fought you because of your faith, nor drove you out of your homes, that you deal kindly and equitably with them. For God loves the equitable.
[51:56] I did not create the Jinn and the humans except to worship Me.
[4:48] God does not forgive that partners be set up with Him, and He forgives lesser sins for whom He pleases. Whoever sets up partners with God has indeed invented a great sin.
[3:91] Those who have rejected and died while they were rejecters, if the earth full of gold were to be ransomed with it would not be accepted from any of them. For these there will be a painful retribution and they will have no supporters.