The Quran

يا أَيُّهَا الَّذينَ آمَنوا توبوا إِلَى اللَّهِ تَوبَةً نَصوحًا عَسىٰ رَبُّكُم أَن يُكَفِّرَ عَنكُم سَيِّئَاتِكُم وَيُدخِلَكُم جَنّاتٍ تَجري مِن تَحتِهَا الأَنهارُ يَومَ لا يُخزِي اللَّهُ النَّبِيَّ وَالَّذينَ آمَنوا مَعَهُ ۖ نورُهُم يَسعىٰ بَينَ أَيديهِم وَبِأَيمانِهِم يَقولونَ رَبَّنا أَتمِم لَنا نورَنا وَاغفِر لَنا ۖ إِنَّكَ عَلىٰ كُلِّ شَيءٍ قَديرٌ

Ali Unal

O you who believe! Turn to God in sincere and reforming repentance.¹ It is hoped² that your Lord will blot your evil deeds from you and admit you into Gardens through which rivers flow, on a Day when God will not disgrace nor disappoint the Prophet and those who believe in his company. Their (the believers’) light will shine and spread before them, and on their right hands, as they say: “Our Lord! Perfect our light (by Your grace, so that we may reach Paradise), and forgive us.³ Surely You have full power over everything!”

‘Ali, may God be pleased with him, the Prophet’s cousin and son-in-law and the fourth Caliph, describes a sincere, reforming repentance as follows:
In order that your repentance can be a sincere, reforming, and valid one, you should (1) sincerely feel remorse for the sin you have committed; (2) fulfill all the obligatory religious duties and make up the missed ones; (3) return any right you have usurped to its owner; (4) beg the pardon of those you have offended; (5) resolve not to commit again the sin you have committed; and (6) make your carnal soul taste the difficulty of obedience to God as you have caused it to taste the pleasure of sinning (Yazır, Zamakhsharī ).
God is never obliged to accept repentance, but a repentant one should sincerely hope that He will accept it and forgive them.
This means that human beings are fallible, and except for the Prophets, almost everyone goes to the other world with some sins. In addition, the good deeds of human beings will not be enough for them to provide the light that they will need in order to reach Paradise from the very moment they will be resurrected. So they will pray to God to complete their light purely out of His grace (also see sūrah 57: 12, 19). They will also need forgiveness in order to be saved from the darkness of their sins. On the other hand, the hypocrites will not be able to advance toward Paradise and will be enveloped by the darkness of their hypocrisy (57: 13).