Sheikh Hamad al-Haydarî, professor at al-Imâm University
A person who relapses from Islam is the same as any other unbeliever.
The obligation of taking a bath (performing ghusl) for the unbeliever who wants to embrace Islam is a matter of disagreement.
Some people of knowledge have said that it is obligatory for a person who embraces Islam to take a bath, since the Prophet (peace be upon him) ordered Qays b. `Asim to perform ghusl using water mixed with the leaves of the lote tree when he embraced Islam. [Musnad Ahmad, Sunan Abî Dâwûd, Sunan al-Tirmidhî and others, and authenticated by al-Nawawî]
Others said it is not obligatory for a new Muslim to take a bath. They argued that had it been obligatory it would be a well-known practice, as there are so many people who embrace Islam all the time.
However, it is safer to follow the first opinion.
The person who relapses from Islam is obligated to pronounce the declaration of faith (shahâdah), the same as anyone else who embraces Islam.
However, the scholars, in regard to the one who relapses, make a distinction with regards to a person whose relapsing from the faith is on account of imposing or legalizing something unlawful. His return to Islam cannot be valid until he gives up his false belief and re-pronounces his shahâdah.
Therefore, he has to remove the reason for his relapsing first, and the restate his shahâdah.
If the person had relapsed and rejected Islam totally, then his return to Islam will be in the same manner as the embracing of Islam by an unbeliever, by pronouncing his shahâdah and acknowledging Islam and its rulings.
And Allah knows best
[See Fiqh al-Shâfi`iyyah (2/224), and for Hanbalî Law, al-Rawd al-Murb` bi-Hâshiyah Ibn al-Qâsim (7/410, 411)]
Source: Islam Today