How Does One Become a Muslim?
In order to become a Muslim, there are no specific religious rituals or
customs that you need to perform, neither in specific areas nor in front of
people. This is because in Islam a person has a direct relationship with his
Lord without any intermediaries.
By simply believing and declaring that there is only one God and that Muhammad is His slave and Messenger one thereby becomes Muslim. Upon making this statement sincerely every sin in his/her life up to that point is forgiven.
God says in the Holy Qur’an Except those who repent and believe and do righteous deeds; for those, God will change their sins into good deeds, and God is Ever Oft-Forgiving, Most Merciful (25:70) You begin life again as a Muslim, one who submits to the will of God.
As for non-Muslims who have accepted Islam, they will be given their reward in double, due to their belief in their messengers as well as their belief in Muhammad.
((Those to whom We gave the Scripture [Torah and the Gospel, etc.] before it, - they believe in it (the Qur’an). * And when it is recited to them, they say: We believe in it. Verily, it is the truth from our Rubb. Indeed even before it we have been from those who submit themselves to Allah in Islam as Muslims. * These will be given their reward twice over, because they are patient, and repel evil with good, and spend (in charity) out of what We have provided them )) (28:52-54)
In addition to this, Allah effaces all the sins they did before accepting Islam. The Prophet said:
“Islam effaces all [the sins one did] before it?” (Muslim).