The Repentance - 9:3
This is also a proclamation from Allah and His Messenger to all the people on the day of the great pilgrimage: Allah is far from the idolaters, and so is His messenger. It is better for you if you repent. And if you turn back, know that you cannot overpower Allah. Announce to the disbelievers a painful punishment.
Why does Allah not forgive those who die associating partners?
The Women/4:48
Allah does not forgive associating partners with Him, but other than that He forgives to whom He wills. Whoever associates partners with Allah has committed a great insult.
The Women/4:116
Allah does not forgive associating partners with Him (if it continues until death), but He forgives the sins below that to whom He wills. Whoever associates partners with Allah has gone astray.
Luqman/31:13
"When Lokman advised his son, he said: "My dear son, do not associate partners with Allah; surely shirk (associating partners with God) is a great wrong."
The Companies/39:65
It has been revealed to you and those before you that if you associate partners, all that you do will be in vain and you will be of the losers.