Some Mormons are visiting him and trying to make him doubt his religion

My dear brother, you have to know with absolute certainty that the religion of Islam is the true religion which is pleasing to Allaah, and that Allaah will accept no other religion from anybody apart from Islam. Islam is based on the worship of Allaah Alone with no partner or associate, and on following the Messenger Muhammad (peace and blessings of Allaah be upon him). This is the implication of bearing witness that there is no god except Allaah and that Muhammad is the Messenger of Allaah. 

If these beliefs are firmly entrenched in the heart of the Muslim, he will not care about any opposition or mockery on the part of others. Rather he will pray to Allaah and act in accordance with the religion of Allaah, and he will say, “I am one of the  Muslims.” Allaah says (interpretation of the meanings):

“And who is better in speech than he who invites (men) to Allaah’s (Islamic Monotheism), and does righteous deeds, and says: ‘I am one of the Muslims.’”

[Fussilat 41:32]

“Therefore proclaim openly (Allaah’s Message Islamic Monotheism) that which you are commanded, and turn away from Al‑Mushrikeen (polytheists, idolaters, and disbelievers).

Truly, We will suffice you against the scoffers,

Who set up along with Allaah another ilaah (god); but they will come to know”

[al-Hijr 15:94-96] 

The Christians are among these mushrikeen because they worship the Messiah and his mother, and they worship the cross. So do not pay any attention to them or their mockery; turn away from them and do not enter into debate with them, lest they make you confused with their specious arguments. If you can change schools and move to a school where you will not be exposed to this trouble, then do so.

 The Mormon sect appears to be Christian because they mention their book, which is the Gospel (New Testament), and they say that if Islam were like this, i.e., opposing their religion, it would have refuted their book. This is ignorance and confusion on their part, for Islam does not refute the Injeel which Allaah revealed to ‘Eesa; in fact the Qur’aan confirms it and the Books that came before it. But Islam refutes the religion of the Christians who have taken the Messiah and his mother as gods instead of Allaah, and Islam refutes the distorted Gospels which are in the Christians’ hands. Allaah says, after mentioning the Tawraat  (Torah) and Injeel (Gospel):

“And We have sent down to you (O Muhammad) the Book (this Qur’aan) in truth, confirming the Scripture that came before it”

[al-Maa’idah 5:48]

i.e., the Qur’aan confirms the Books which came before it, such as the Tawraat and Injeel.

The Messiah has disowned these mushrik Christians and their way, and he will disown them on the Day of Resurrection, as Allaah says (interpretation of the meaning):

“Surely, they have disbelieved who say: ‘Allaah is the Messiah [‘Eesa (Jesus)], son of Maryam (Mary).’ But the Messiah [‘Eesa (Jesus)] said: ‘O Children of Israel! Worship Allaah, my Lord and your Lord.’”

[al-Maa’idah 5:72]

“And (remember) when Allaah will say (on the Day of Resurrection): ‘O ‘Eesa (Jesus), son of Maryam (Mary)! Did you say unto men: ‘Worship me and my mother as two gods besides Allaah?’  He will say: ‘Glory be to You! It was not for me to say what I had no right (to say). Had I said such a thing, You would surely have known it. You know what is in my inner‑self though I do not know what is in Yours; truly, You, only You, are the All‑Knower of all that is hidden (and unseen).

Never did I say to them aught except what You (Allaah) did command me to say: “Worship Allaah, my Lord and your Lord.” And I was a witness over them while I dwelt amongst them, but when You took me up, You were the Watcher over them; and You are a Witness to all things.’”

[al-Maa’idah 116-117]

We ask Allaah to make you steadfast in Islam and to suffice you against the evil people. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.