Falling into cardinal sins, any chance for repentance?

What would stop him from being forgiven? This is the short answer to your question based on a similar reply that occurs in a Hadith, which is directly related to your question. I will give you the Hadith first and then speak about your question in more detail. The Hadith explains that God's mercy is available to human beings at all times, no matter how grave the sins they have committed. It gives this message in a symbolic story.

The Hadith speaks of a man who had killed ninety-nine people. He then began to think of his position and feared God's punishment. So he resolved to mend his ways and to seek God's forgiveness, if that was at all possible. He began to inquire about a priest or a learned man who could help him achieve his purpose. He was guided to an old priest whom people considered to be the most learned on earth. He went to him and knocked on his door. The old man opened the door and inquired what he wanted. The man said: "I have killed ninety-nine people and I want to repent. Can I have my repentance accepted?" The old man said: "How do you think your repentance could be accepted after having killed ninety-nine people?" That answer filled the man with despair and he killed the priest on the spot.

The man was not to be deterred. Something within him urged him to try again. He began to ask again about the most learned person. People told him of another priest who was highly respected among them. He went to him and knocked on his door. An old man with a long white beard appeared and asked his purpose. The man said: "I have killed one hundred people and I want to repent. Do you think my repentance would be accepted?" The old man said: "What could stop you for having your repentance accepted? It certainly will if God knows that your repentance is genuine and sincere."

The old man talked to his visitor for a while and then suggested to him that he had a better chance of making his repentance sincere if he lived in a town where the people were good and religious. He also told him that if he were to continue to live in a city where he committed all those killing, he was bound to have all sorts of diversions, which might take him away from his purpose of genuine repentance. The man left and took his way directly toward the town recommended by the priest.

While on his way, the man was taken ill and died. A group of the angels of mercy and a group of the angels of punishment arrived at the spot where his body lay, each wanted to take him away. The angels of punishment said: "He had not done a single good act." The angles of mercy replied: "He has undertaken this trip full of repentance, determined to be obedient to God." They were disputing his case between themselves, when God sent them an angel. Both groups agreed to make that angel an arbiter. When they put the case to him, he said: "I suggest you measure the distance between the land of evil and the land of goodness. If he is found to be closer to the land of goodness, then the angels of mercy should take him. But if he is found to be closer to the land of evil, then the angels of punishment may have him." Both groups were happy with this judgment and they started measuring the distance to each of the two towns.

The Hadith concludes with this statement. "God commanded the land of goodness to draw near and commanded the land of evil to draw further away. The angels found him closer to the land of goodness and he was taken by the angels of mercy."

This Hadith tells it all. There is nothing to stop any person, no matter how grave his past sins, from turning a new page and starting a new, good life, which wins him God's forgiveness. The final statement in the Hadith is particularly revealing. It was by God's command that the man was found closer to the land of mercy. This means that had he taken only a couple of steps on the way before his death, he would have still been found closer to the land of goodness and he would have been forgiven. The measuring of distances was not the determining factor. That factor was God's knowledge that the man was sincere.

God's forgiveness is available to us all, even the most hardened sinners among us. This is clearly stated in the Qur'an and in many Hadiths. In the Qur'an God says:
"God will never forgive that partners are associated with Him. He may forgive anything else to whomever he pleases." [Woman — “An-Nis'a” 4: 48]

This is a definitive statement from which we can outline the requirements of forgiveness of all sins, grave and small.

The first condition is that of faith. God may forgive a believer, but He may not forgive even the smallest of sins to a person who associates partners with Him. Hence, before we ask God's forgiveness, we have to make sure that we truly believe in His Oneness, recognizing His attributes and believing in every single one of them. Secondly, our repentance of past sins must be genuine.

This is accomplished with a sincere resolve that we will not return to those sins whatever may be the temptations. If we happen to slip again, we immediately renew our repentance and strengthen our resolve. Finally, we should take all possible means and measures to help us fulfill our determination. Just like the old priest who pointed out to the man in the Hadith when he suggested to him to move to the land of goodness. There he would find people to welcome him and to be associates in seeking God's forgiveness. He would have less temptation to go back to his past ways.

Sins and offenses are of two types; those, which concern our relationship with God, and those that concern our relationship with fellow human beings. If a person repents after having committed sins and offenses of the first type, such as neglecting his duties toward God, or committing offenses which run contrary to what God requires and then he genuinely repents and seeks God's forgiveness, God will forgive him those straightway. Offenses that are committed against the rights of other people have a different approach. In this category you have offenses such as theft, cheating, usurping other people's rights, ill treatment of people, backbiting, etc. God will not forgive these offenses till the people who suffered as a result of them agree to forgive the offender. On the Day of Judgement, everyone is brought face to face with every other person who has a claim against him. God will ask the offender to compensate the person who had suffered as a result of the offense until he or she is satisfied. That takes the form of taking away some of the good deeds of the offender and crediting them to the offended person. If the offender does not have such good deeds, then some of the bad deeds or sins committed by the offended person will be taken away from him and added to the offender. Either way will enhance the offended person's chances of being admitted to heaven.

A different method of compensation takes place if God is satisfied with the repentance of the offender and He chooses to help him. In this case, God decides to take over the task of satisfying the offended person, either by giving him some of his sins, or by increasing his reward for his good deeds until he is satisfied and forgoes his claim against the offending person.

This applies to any rightful claim by other people against us. As you realize, this applies to offenses that went unpunished or uncompensated for in this life and left to the Day of Judgement. If a person has committed an offense and got punished for it, according to the Islamic law, as in the case of a theft, then the punishment is regarded as compensation for the crime. The other way is for the offender to go to the person who has been offended against and return to him what is rightfully his. Thus, if someone has stolen something from another, and he returns the stolen thing and sought forgiveness, then he stands a good chance of being forgiven by God. It is far better to do this than to leave matters to the Day of Judgement, when one has to pay out of his reward for his good deeds in order to pay off what belongs to others. If this is impossible because one does not know how to get to the people who have a claim against him, or if he tries to satisfy them for the offenses, he may run into great problems, then he could leave the matter to God to settle it on his behalf on the Day of Judgement, provided that he tries hard to increase his reward by every means. For example, if he has unlawful monetary gains from others and he cannot repay them, either because he does not know them, or because he will be facing enormous problems, then he should pay to charity what he has gained from them or even more. He could also improve his worship and be always available to serve the cause of Islam. He will then stand a good chance of earning forgiveness.