The Prophet Muhammad peace be upon him said about a statement that Allah SWT spoke about the aspects of forgiveness. He SWT said “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 as it.”
This is a very profound and merciful attribute that is part of the abundant mercy and forgiveness that Allah SWT has for humanity. Allah SWT informs the people that the aspect of asking and calling out to Allah for the mistakes and sins that they have committed and every human that is alive today makes mistakes and wrongs themselves by committing sins. That if they ask Allah for forgiveness for the mistakes that they make, repenting to Allah and through this calling and seeking Allahs forgiveness for their mistakes, then surely with absolute certainty Allah who is the most merciful will forgive that person and it is mandatory to think that when a person sincerely is repentant and asks Allah for forgiveness then that person has been forgiven by Allah. Such a situation a person has to try and not repeat the mistakes and also even if their sins were to reach the clouds above the sky or even fill the entire the earth with their sinful activities, if they sincerely seek the face of Allah ascribing no partners to Allah in other words maintaining and remaining firm upon tawhid the oneness of Allah believing that only Allah can forgive them then with certainty Allah SWT will forgive that person no matter what evil sinful action they have committed. The key to this understanding is that the only sin that is not forgiven is shirk in other words ascribing partners or divinity to some aspect of creation like how pagans worship idols and even people of the book worship Isa jesus peace be upon him and jews worshipped Uzair peace be upon him from some of their sects. So any person that ascribes partners and divinity to any aspect of creation then they have committed the greatest sin that Allah does not forgive unless they repent on this earth and free themselves from this injustice and wrongdoing of ascribing partners to Allah SWT. So any person that sincerely understands the fundamental truth of the shahada that there is no God worthy of worship except Allah and within their hearts remains firm upon this, then even if they have committed the worst sins like murder, theft, rape, oppression to parents, then no matter what evil they have done if they repent and reform and seek Allah forgiveness then Allah will forgive them, however they will upon the earth have to bear with the consequences of the sins that they have committed and as a penalty may also be facing trials and tribulations from people that know of their sinful lives for public declaring and letting others know of your sins is a sin itself, yet all sins even shirk can be forgiven however it requires a person to remain firm upon tawhid in their hearts and they verbally confirm the oneness of Allah and they seek to reform, repent and not repeat the mistakes, and even if they make other mistakes or any repeat of the mistake if they sincerely keep asking Allah for forgiveness trying not to repeat the mistake if they still fall into error, that their sins accumulate to as much as the size of the earth, Allah will still forgive them as Allahs forgiveness is greater than his wrath.
It is important to understand that without having tawhid firm upon the heart and living by it, and if a person dies on the earth without the understanding and belief in the oneness of Allah they will not be forgiven for the shirk they commit. The exception is the insane and those that cannot due to physical or mental disability like some person who is deaf, dumb and blind and cannot sense or understand life due to their illness or disability they will not be held accountable as the pen is lifted from the insane until sanity reaches them again. So the aspect of tawhid and sincerely calling and asking Allah for forgiveness no matter what a person has done no matter what evil or sinful actions they have accumulated all can be forgiven by Allah as this is the one of the most greatest attributes of Allah that he is most forgiving, all forgiving, continues to forgive and continuously forgives humanity for their mistakes. As some of the names of Allah like Al Ghafoor and Al Ghafir and Ar Raheem the meaning is the one who forgives, continuously forgives and is the most merciful. Whereas people are not so quick or even close to forgiving what some sins that humanity has perpetrated, and it is well known saying of one of the righteous companions of Prophet Muhammad peace be upon him that the best revenge is to forgive them. However most people their hearts are too hard and hard heartened people suffer and cause suffering due to the nature of hearts which can be harder than stone resulting in a person unable to understand, think, believe, hear, or see the greatness of the mercy and forgiveness of Allah SWT. So seek the forgiveness of Allah SWT and abstain from shirk every day and night till you reach the certainty of death and you will be forgiven.