Please help me in converting someone to Islam after he left Islam.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Apostasy is the most major sin that a person could commit; we seek refuge in Allah from that.  Apostasy nullifies good actions as Allah Says (interpretation of meaning): {…"If you join others in worship with Allâh, (then) surely (all) your deeds will be in vain, …}[39:65].

In addition to this, if someone apostates and dies on that state, he will be severely punished and be eternally in Hellfire.

Allah Says (interpretation of meaning): {Verily, Allâh forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, …}[4:48].

In Saheeh Muslim it is reported that Jabir (Radiya Allahu Anhu) narrated: “A man came to the Prophet (Sallallahu Alaihi wa Sallam) and said: What are the two things that necessarily lead to (Paradise)?  The Prophet (Sallallahu Alaihi wa Sallam) replied: “Whoever dies not having associated anything with Allah will be admitted to Paradise.  And whoever dies having associated anything with Allah will enter Hellfire.”

As for the manner of convincing an apostate from Islam, this differs from one individual to another.  First, we have to find out the reasons of his apostasy, and correct any misunderstandings or misconceptions that he might have.  We have also to remind him about what Allah has prepared for his pious slaves, and what he has prepared for those who have gone astray from His correct path.

We have to choose the appropriate time i.e. when he is not preoccupied and can pay attention, we should call him (to Islam) with wisdom, admonish him, and argue with him with the best manners.  We also have to ask Allah to guide him, and help him materially if that would benefit him. We may also use any other manners and methods of Da’wah that might be of some help in his situation.