Every Sin is Pardonable

According to the verse of the Holy Quran and Mutawatir (widely related) traditions all the sins which man commits are pardonable if he repents sincerely. It is wrong to say that such a sin can never be pardoned. The Almighty Allah says in the Glorious Quran, "And He it is who accepts repentance from His servants and pardons the evil deeds and He knows what you do." (Surah Shura 42:25)

Allah has mentioned His names as:

1. Tawwabun (one who accepts Tawbah)
2. Gaffarun (one who pardons)
3. Ghafoorun (one who forgives totally)
4. Ghaaferuz Zanbe (Pardoner of sins)
5. Qaabelut Tawbah (one who accepts repentance)

Allah has given a general invitation to all the people to turn towards Him and seek forgiveness for their sins. If we study the meaning of the 54th verse of Surah Zumar we find that sinners have no cause to lose hope. This Ayat is also known as Ayat-e-Rehmat (The verse of Mercy).
"Say: O my servants! who have acted extravagantly against their own souls, do not despair of the Mercy of Allah ; Surely Allah forgives the faults altogether; surely He is the Forgiving, the Merciful." (Surah Zumar 39:53).

Unlimited Grace
A few points must be noted from the above verse. Firstly, Allah says, 'O my servant' and not 'O sinners!' Even though he is addressing the sinners he addresses them as 'my servants'. So that the hopelessness of the servants may be transformed into hope in His Mercy.
Secondly, he says, "Who have acted extravagantly." This denotes a soft approach. Allah does not say 'O you who have acted blatantly,' So that the sinners may not lose hope of obtaining pardon.

Ref: Greater sins, Vol.1