All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The term Laa Ilaaha Illallaah means that none has the right to be worshipped but Allaah, this means denying the existence of any true Illaah (God) other than Allaah, and confirming that the only true Illaah is Allaah Alone, Exalted is He.
If a person says "Ash-Hadu Allaa Ilaaha Illallaah'' (I testify that none has the right to be worshipped but Allaah) this means that he confirms, acknowledges and believes that none has the right to be worshipped but Allaah, as He is The Great and Almighty. It is not permitted to dedicate any act of worship to other than Allaah, and whoever dedicates them to other than Allaah goes out of the fold of Islam.
As regards the questions ''why should a person be a Muslim?'', and ''what distinguishes Islam from other religions?'', then the answer is that every person is required to be a Muslim and that Islam is the religion of Allaah. Allaah ordered all the people to follow it and He does not accept any other religion than it. Allaah Says (what means): {And whoever desires other than Islam as religion – never will it be accepted from him, and he, in the Hereafter, will be among the losers.} [Quran 3:85].
Islam is also the religion of all the Prophets and Messengers, may Allaah exalt their mention, as we discussed in Fatwa 88906.
What distinguishes Islam from other religions, along with being a true religion from Allaah and the religion of all the Prophets and Messengers, may Allaah exalt their mention, is that it corresponds to the Fitrah (natural disposition upon which Allaah created mankind) which makes a person surrender and submit to its Creator.
Islam corresponds to the requirements and needs of the people, and organizes their affairs, contrarily to other religions that are invented by man and which are full of contradictions and things that go against Fitrah, and lead him to hardship and difficulties, or to dissoluteness and deviation. Allaah Says (what means): {[And say, “Ours is] the religion of Allaah. And who is better than Allaah in [ordaining] religion? And we are worshippers of Him.”} [Quran 2:138]
For more benefit, please refer to Fataawa 99189, 91689, 81979and 82037.
Finally, we advise you to treat your new Muslim brother kindly, and you should teach him the religious principles and the pillars of Islam. Moreover, you should take him to the centres which take care of new Muslims.
We ask Allaah to greatly reward you on the Day of Judgment for being the reason of making this new Muslim steadfast on the religion of Allaah.
Allaah Knows best.