In the name of Allah, Most Gracious, Most Merciful. Surah 1:1
The Koran begins by calling Allah compassionate, merciful and gracious.
The Koran uses the word “gracious,” “merciful” and “compassionate” over 200 times in reference to Allah. The Bible similarly describes God in these same terms.
Both books make it very clear that God is both merciful and just. He shows mercy but He condemns the unrighteous. The question is this: do both books describe a similar level of graciousness, compassion and mercy?
The Koran shows limits to Allah’s forgiveness. For example, if one turns to other gods, Allah will not forgive you. Ever.
Allah forgiveth not (The sin of) joining other gods with Him; but He forgiveth whom He pleaseth other sins than this: one who joins other gods with Allah, Hath strayed far, far away (from the right). Surah 4:116
Allah suggests some very unmerciful punishments for those who sin.
They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them; and (in any case) take no friends or helpers from their ranks. . . Surah 4:89
And (as for) the man who steals and the woman who steals, cut off their hands as a punishment for what they have earned, an exemplary punishment from Allah; and Allah is Mighty, Wise. But whoever repents after his iniquity and reforms (himself), then surely Allah will turn to him (mercifully); surely Allah is Forgiving, Merciful. Surah 5:38-39
The Koran says that Allah has limits to his forgiveness, especially toward evil people.
Whether thou ask forgiveness for them or ask not forgiveness for them is all one for them; Allah will not forgive them. Lo! Allah guideth not the evil-living folk. Surah 63:6
And it appears Muhammad had limits to his own mercy, as seen in this chapter directed towards his own uncle, who opposed Muhammad.
Perish the hands of the Father of Flame! Perish he!
No profit to him from all his wealth, and all his gains!
Burnt soon will he be in a Fire of Blazing Flame!
His wife shall carry the (crackling) wood - As fuel!-
A twisted rope of palm-leaf fibre round her (own) neck! Surah 111
In contrast to the Koran, no where in the Bible does it say that God will not forgive a sin. All sin, all unrighteousness is forgivable—murder, stealing, lying, sexual impropriety. All the person must do is ask for forgiveness.
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. 1 John 1:9
God forgave the Israelites when they worshipped the golden calf. He forgave the nation when they worshipped pagan idols. He forgave Moses for murder. He forgave David for infidelity. There were consequences to all those sins, definitely, but God forgave those who asked for forgiveness. There are no limits to the mercy God shows in the Bible.