Allah which means (God), is the most merciful/forgiving.

People who ask for forgivness from Allah the Oft‑Forgiving, and Most Merciful. No matter how great the sin, the forgiveness, generosity and kindness of Allah is greater.

Allah says: Say: "O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful". Quran (39:53).

If a person repents sincerely then Allah will accept his repentance and forgive him his sins; meaning that even if you have committed shirk (ascribing to Him any partner) than you turn back from your sin and ask forgiveness from Allah, Allah is willing to forgive you for your sin as long as you sincerely turn back from what ever sin you have committed and do good - as long as you always turn back from your sin rather it is a small or a huge sin Allah will forgive you because you have came back to God; the Creator. If you don’t turn back from the sin you have committed rather it is major like shirk (ascribing to Him any partner) or the smallest sin you can think of than your not forgiven for it if you havent recognized your mistake and turn back and ask for forgiveness from the one who is the All Forgiving Allah; your Creator. As long as you have repented to Allah before death you will be forgiven for all that you have repented no matter how great the sin was.

Allah which means (God), is the most merciful/forgiving. Allah forgives his servants when they ask Him sincerely for forgiveness from the sin/sins they have done and not intentionally redoing the same sin again.

Allah says in the Holy Quran: "And He it is Who accepts repentance from His slaves, and forgives sins, and He knows what you do". Quran (42:25)

To be sincere and ask Allah to forgive you and to repent from the sin is what you must do to obtain forgiveness from Allah. 'When Allah sees this sincere repentance from one of His servants – a servant who truly turns to his Lord in fear and hope – He not only forgives the sin, but replaces those sins for good deeds to the servant’s credit. This is from Allah’s infinite grace and munificence.' 'Allah also forgives us our sins through the difficulties that we face in life. When we are stricken with illness or suffer from circumstances, we will earn forgiveness if we bear them patiently seeking Allah’s reward.' Thats how merciful Allah is. 'Above and beyond all of this is the mercy of the Most-Merciful Lord who pardons on that Day all sins as He pleases, as long as the person meets Him worshipping Him alone without ascribing to Him any partner.'

"The repentance accepted by Allah is only for those who do wrong in ignorance [or carelessness] and then repent soon after. It is those to whom Allah will turn in forgiveness, and Allah is ever Knowing and Wise. But repentance is not [accepted] of those who [continue to] do evil deeds up until, when death comes to one of them, Allah says: "Indeed, I have repented now," or of those who die while they are disbelievers. For them We have prepared a painful punishment." Quran 4:17

For any (Christian, atheist ,Buddhist, Jew, Hindu) who accepts Islam, all his/her previous sins will be forgiven. Moreover, God will change their sins into good deeds. This is amazing:

Allah says: "Except those who repent and believe (in Islamic Monotheism), and do righteous deeds for those, Allah will change their sins into good deeds, and Allah is Oft-Forgiving, Most Merciful." [Quran 25:68-70].

if you really regret what you've done in the past then go repent now... one of allah's (SWT) 99 names is "the forgiver". And he will forgive you.

Few verses regarding repentance and how to repeat sincerely. Allah can forgive anything if a person repents sincerely.

Allah (SWT) Most High says in Noble Qur'an, "Ask your Lord for forgiveness and then turn in repentance to Him..." (11:3)

Also Allah (SWT) Most High says in Noble Qur'an, "O you who believe! Turn (in repentance) to Allah with sincere repentance; Perhaps your Lord will remove your evil from you... " (66:8)

"And seek forgiveness of Allah; surely Allah is Forgiving, Compassionate." (Qur’an 73:20)

"And whoever does evil, or wrongs his own soul, but afterwards seeks Allah’s forgiveness, will find Allah is Forgiving, Compassionate." (Qur’an 4:110)