One unpardonable sin according to Islamic perception is being a polytheist.

"Allah does not forgive that partners be associated with Him, and He forgives anything beneath that to whomever He wills." (Nisa 4:48)

"Allah forgive not joining other gods with Him; but He forgive whom He please other sins than this: one who joins other gods with Allah, has strayed far away." (Nisa 4:116)

The central theme of the Quran is of God's unity and his greatest attribute of mercy.

"My mercy has encompassed every thing." (Al-A'raf 7:56)

"Tell them (O Muhammad): 'O My slaves who have wronged their souls, do not despair of Allah's mercy, for Allah will forgive all the sins. He is indeed very Forgiving, very Compassionate, Turn to Him then, and obey Him (in all your affairs).'" (Zumar 53:54)

Qudsi Hadeeths in support of God’s mercy.

"When Allah decreed the creation He pledged Himself by writing in His book which is laid down with Him: My mercy prevails over my wrath."

"Says Allah, 'He who knows (and believes) that I forgive all sins, then I do forgive all his sins, and do not care - so long as he does not suggest partners unto Me."

"A man said: By Allah, Allah will not forgive So-and-so. At this Allah the Almighty said: Who is he who swears by Me that I will not forgive So-and-so? Verily I have forgiven So-and-so and have nullified your own good deeds (or as he said it)."

"O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you. O son of Adam, were you to come to Me with sins nearly as great as the earth and were you then to face Me, ascribing no partner to Me, I would bring you forgiveness nearly as great at it."

"The gates of Paradise will be opened on Mondays and on Thursdays, and every servant of Allah who associates nothing with Allah will be forgiven, except for the man who has a grudge against his brother. About them it will be said: Delay these two until they are reconciled; delay these two until they are reconciled."

Residents of hell will get out of hell because of Allah's mercy, nullifying common misconception of staying in hell for eternity, as supported by Quran (7:40) and Matthew 19 23-26, it is easier for a camel to go through the eye of a needle.