It is very clearly mentioned that God can forgive sins. It is repeatedly mentioned that God forgives the sins of his worshipper. God forgives the sins done by the father. God even forgives/destroys the most heinous sins. God can forgive all sins done unknowingly. God can forgive all the sins done in the past or done currently.
Only a True Guru (Satguru) who gives the True Naam (Satnaam) of God can help one get rid of the previous sins by giving true and correct worship in accordance with our Holy Scriptures.
Kabir Sahib says:
कबीर,जबही सत्यनाम हृदय धरयो,भयो पापको नास।
मानौं चिनगी अग्निकी, परी पुराने घास।।
The True Name (satnaam) of God can destroy sins like fire destoys dry grass.