A Christian Asking About Repentance
Question

If a man decides after a life of sinfulness to come to God and promise to reform his life, I understand that Islam says that he will be forgiven and therefore inherit the promises etc.
My question though is what has happened to the weight of this man's sin, as in sinning he has offended God and therefore this sin has to be atoned for; who is going to atone for his sin, if God is just going to forgive him on the promise of his being good and faithful? Surely as we saw with Adam there has to be a consequence for sin...
Praise be to Allah.Who has guided us to Islam, for we would not have been guided had He not guided us. 
First of all, we thank you for your question, and we ask Allah to bless you with guidance. 
Allah has created us for a great purpose, which is to worship Him Alone with no partner or associate. Allah says (interpretation of the meaning): 
“And I (Allah) created not the jinn and mankind except that they should worship Me (Alone)” [al-Dhaariyaat 51:56]
This is the purpose of life: to worship Allah alone. 
So Allah has not created us to eat and drink… or to relax and play… or to strive and work hard… rather He has created us to worship Him and not to be ungrateful to Him; to remember him and not forget Him.  
This is the purpose of life, and what a beautiful purpose it is. When man lives to worship his Lord and Master, to serve His religion and establish His command, with his body on earth and his heart with Allah and the Hereafter, then he understands the reality of this life and how insignificant this world is, and that what is left of this life should not be wasted in passing pleasures and temporary desires. O Allah, bless us with Your guidance.
Because worship requires explanation and guidance, Allah sent His Messengers: 
“Messengers as bearers of good news as well as of warning in order that mankind should have no plea against Allah after the (coming of) Messengers” [al-Nisa’ 4:165]
Whoever submits is guided, and whoever turns away is doomed and lost. 
Allah says (interpretation of the meaning):
“Verily, proofs have come to you from your Lord, so whosoever sees, will do so for (the good of) his ownself, and whosoever blinds himself, will do so to his own harm, and I (Muhammad) am not a watcher over you” [al-An’aam 6:104]
If a person submits (becomes Muslim), he has chosen happiness for himself: 
“And whosoever has embraced Islam (i.e. has become a Muslim by submitting to Allah), then such have sought the Right Path”
[al-Jinn 72:14 – interpretation of the meaning]
Because man is vulnerable to making mistakes and forgetting, and to falling into sin, Allah has prescribed repentance for His slaves and has opened the door of repentance until the Hour begins, and He invites His slaves to repent sincerely. Allah says (interpretation of the meaning): 
“O you who believe! Turn to Allah with sincere repentance! It may be that your Lord will expiate from you your sins, and admit you into Gardens under which rivers flow (Paradise)…” [al-Tahreem 66:8] 
“And all of you beg Allah to forgive you all, O believers, that you may be successful” [al-Noor 24:31]
See also question no.  14289.
 It should be noted that sins are of two types: 
1 – Transgressions against the rights of Allah. 
2 – Transgressions against the rights of created beings. 
With regard to the first type: 
Transgressions against the rights of Allah – such as adultery, drinking alcohol, neglecting obligatory duties such as prayer, zakaah, etc. These sins are punishable according to sharee’ah, such as adultery and drinking, for which the hadd punishment is to be carried out on the one who does them, and that is an expiation and a purification for him. If the hadd punishment is not carried out on him, but he repents to Allah, then Allah will accept his repentance and turn his bad deeds (sayi’aat) into good deeds (hasanaat). 
Whoever meets Allah with these sins, without having repented or had the hadd punishment carried out on him, is subject to the will of Allah on the Day of Resurrection: if He wills He will punish him and if He wills He will forgive him. 
Al-Bukhari (18) and Muslim (1709) narrated from ‘Ubaadah ibn al-Saamit (may Allah be pleased with him) – who was present at the Battle of Badr and was one of the prominent figures on the night of al-‘Aqabah – that the Messenger of Allah (peace and blessings of Allah be upon him) said, when there was a group of his companions around him: “Give me your oath of allegiance (bay’ah), pledging that you will not associate anything with Allah, you will not steal, you will not commit adultery, you will not kill your children, you will not fabricate lies and you will not disobey with regard to anything that is right and proper. If you fulfil that, your reward will be with Allah, but whoever commits any of these actions, his punishment will be in this world and it will be an expiation for him. Whoever does any of these things then Allah conceals him, it will be up to Allah: if He wills He will forgive him and if He wills He will punish him.” So they gave their oath of allegiance to him on that basis. 
According to a report narrated by al-Bukhari (6416): “Whoever does any of these things will be punished in this world and that will be an expiation for him and a purification.” 
Al-Haafiz said in al-Fath (1/68): It is understood from this hadeeth that carrying out the hadd punishment is an expiation for sin, even if the person who is punished does not repent. This is the view of the majority of scholars… 
Ahmad (1365) narrated that ‘Ali (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever commits a sin in this world and is punished for it, Allah is too just to punish His slave a second time. Whoever commits a sin in this world then Allah conceals that and forgives him, Allah is too generous to go back to something that He has forgiven.” 
This hadeeth was classed as hasan by al-Arnaa’oot in Tahqeeq al-Musnad, and al-Haafiz classed a similar report from al-Tabaraani as hasan.  
Allah says (interpretation of the meaning): 
“And those who invoke not any other ilaah (god) along with Allah, nor kill such person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.
The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;
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” [al-Furqaan 25:68-70]
“Verily, Allah forgives not (the sin of) setting up partners (in worship) with Him, but He forgives whom He wills, sins other than that, and whoever sets up partners in worship with Allah, has indeed strayed far away.” [al-Nisa’ 4:116]
This verse has to do with the one who does not repent. He is subject to the will of Allah, unless he commits shirk (the sin of associating other with Allah), because shirk cannot be forgiven. 
With regard to the second type of sins:                                      
Transgressions against the rights of people, such as transgressing against their wealth by stealing it or seizing it by force, etc., or transgressing against their honour by means of gossip and slander, or harming them physically by striking them, etc. Repentance from this type of sin is subject to the condition that the rights be restored to the people, or they agree to forgive the offender. 
Whoever does not do that will remain subject to the effects of his sin until the Day of Resurrection, when an amount commensurate with his offence will be taken from his good deeds (hasanaat), as the Prophet (peace and blessings of Allah be upon him) said: “Whoever has wronged anyone with regard to his honour or anything else, let him settle the matter today before there is no dinar and no dirham, and if he has any righteous deeds, an amount commensurate with his wrongdoing will be taken from him, and if he has no good deeds (to his credit) some of the bad deeds of his companion will be taken and added to his burden.” Narrated by al-Bukhari, 2317. 
From this we know that the view that the sinner must be punished in this world is a view for which there is no evidence, but if a person is punished (in this world) that will be an expiation for him, and if he is not punished then he repents from his sin, Allah will accept his repentance. 
Even more false than this is the view of those who say that the punishment for sin may be borne by someone other than the sinner, as some ignorant people say concerning Adam (peace be upon him), claiming that his progeny – including the Prophets – bore the burden of his sin until God sent down His only son to be crucified and slain to free the world of sin! This is a lie and a fabrication against Allah and against His Prophets, and is a kind of injustice of which the divinely-revealed laws are free, because Allah does not punish anyone for the sins of another. Allah says (interpretation of the meaning): 
“And no bearer of burdens shall bear another’s burden” [Faatir 35:18]
Allah is too merciful and too just to punish the offspring for the sins of their father, especially since he repented and Allah accepted his repentance. 
Allah says (interpretation of the meaning): 
“Then the Shaytaan (Satan) made them slip therefrom (the Paradise), and got them out from that in which they were. We said: ‘Get you down, all, with enmity between yourselves. On earth will be a dwelling place for you and an enjoyment for a time.’
Then Adam received from his Lord Words. And his Lord pardoned him (accepted his repentance). Verily, He is the One Who forgives (accepts repentance), the Most Merciful” [al-Baqarah 2:36-37]
“Then they both ate of the tree, and so their private parts became manifest to them, and they began to cover themselves with the leaves of the Paradise for their covering. Thus did Adam disobey his Lord, so he went astray.
Then his Lord chose him, and turned to him with forgiveness, and gave him guidance” [Ta-Ha 20:121-122]
So two things apply in the case of Adam: he was punished for his sin, and he repented from it and Allah accepted his repentance, and He chose him and honoured him. 
The point is that whoever lives a life filled with disobedience and sin only has to turn to his Lord, the most Merciful and Most Generous, and seek His forgiveness and repent, in order for Allah to accept his repentance, as Allah has promised (interpretation of the meaning): 
“Say: O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily, Allah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful” [al-Zumar 39:53]
This is part of the kindness that Allah has made in this tolerant sharee’ah. It was decreed for the Children of Israel that the innocent among them kill the wrongdoers so that their repentance would be accepted [cf. al-Baqarah 2:54], then Allah lifted this heavy burden from this nation on whom He has had mercy.  
Finally we ask Allah to bless the questioner and guide him and open his heart to Islam, so that he may become a member of the Muslim ummah (nation) which accepts Allah as its Lord, Islam as its religion and Muhammad as its Prophet. 
And Allah knows best.