Taubah/ Repentancein Islam

Surah Nisaa' - Repentance - Verses 17,18;
(4:17) (And remember that) Allah's acceptance of repentance is only for those who commit evil out of ignorance and then soon repent. It is towards such persons that Allah turns graciously. Allah is All-Knowing, All-Wise. (4:18) But of no avail is repentance of those who do evil until death approaches any one of them and then he says: 'Now I repent.' Nor is the repentance of those who die in the state of unbelief of any avail to them. For them We have kept in readiness a painful chastisement.27

Notes
27. The Arabic word tawbah means 'to return, to come back'. A man's tawbah after he has sinned means that God's servant, who had turned away from his Master in disobedience, has repented, and has returned to obedience and service. On the other hand, tawbah on the part of God means that the attention of the Master, which had turned away from His erring servant, has once again turned towards him. In this verse, however, God makes it clear to His servants that tawbah is acceptable only from those who commit errors inadvertently and out of ignorance. Such persons will always find the door of God open for them whenever they turn to Him in repentance.

But this tawbah is not for those who pile sin upon sin throughout their lives in sheer indifference to God and who cry for pardon as soon as they see the angel of death approaching. The Prophet (peace be on him) has warned against this attitude in the following words: 'God accepts the repentance of a slave until the gurgling (of death) begins.' (Tirmidhi, 'Da'wat', 98; Ibn Majah, 'Zuhd', 30; Ahmad b. Hanbal, Musnad, vol. 2, pp. 132 and 153, and vol. 3, pp. 425 - Ed.) For when the last leaf of a man's book of life has been turned, what opportunity remains for a man to return to righteous conduct? Likewise, if a person spends even the very last moment of his life in a state of disbelief and then on the threshold of the Next Life he comes to discover that the facts are quite contrary to what he had imagined, what sense is there for him to seek forgiveness?