Answer
This is because Allaah has declared that Jews and Christians are disbelievers in His Book. Allaah has said,
"And the Jews say, 'Ezra is the son of Allaah,' and the Christians say, 'The Messiah is the son of Allaah.' This is a saying from their mouths. They imitate the saying of the disbelievers of old. Allaah's curse be on them, how they are deluded away from the truth. They took their rabbis and their monks as lords besides Allaah and [they also took as Lord] the Messiah, son of Mary. But they were commanded only to worship none but One God. Praise and Glory be to Him, [far above is He] from having the partners they associate with Him." [at-Taubah 9:30-31]
That shows that they are polytheists who associate partners with Allaah. In other verses, Allaah has made it clear that they are disbelievers:
"Surely in disbelief are they who say that Allaah is the Messiah, son of Mary." [al-Maidah 5:17 and 72]
"Surely, disbelievers are those who said, 'Allaah is the third of the three [in a Trinity]." [al-Maidah 5:73]
"Those among the Children of Israel who disbelieved were cursed by the tongue of David and Jesus, son of Mary." [al-Maidah 5:87]
"Those who disbelieve among the People of the Book and the idolaters will abide in the Fire of Hell." [al-Bayyinah 98:6]
Many verses and hadith express the same meaning. The one who rejects the idea that the Jews and the Christians who do not believe in Muhammad [peace be upon him] and deny him are disbelievers is, in fact, denying what Allaah has said. Denying what Allaah has said is blasphemy. If anyone has any doubt concerning them being disbelievers, then he himself is also a disbeliever.
Glory be to Allaah! How could that man be pleased to say that it is not allowed to say that they are disbelievers when they themselves say that Allaah is one of a Trinity? Their Creator Himself has declared that they are disbelievers.
How could this man not be pleased to call them disbelievers when they actually say, "Jesus is the son of God" or "the hand of Allaah is tied," and "Allaah is poor and we are rich"?
How could this man not be pleased to call them disbelievers and apply the word of disbelief to them in general when they describe their Lord in such ways that are nothing but cursing and abusing Allaah?
I call upon that man to repent to Allaah and to read Allaah's words,
"They wish that you should compromise (in religion out of courtesy) with them, so that they (too) would compromise with you." [al-Qalam (68):9] He should make it clear to everyone that they are disbelievers and that they are the inhabitants of the Hell-fire. The Prophet (peace be upon him) said,
"By the One in whose hand my soul is, no Jew or Christian of this community hears about me - that is from the community of people from the time of the Prophet (peace be upon him) until the Day of Judgement - and then does not follow me - or he said does not believe in what I brought - except that he is from the inhabitants of the Hell-fire." [Saheeh Muslim]
The one who made such a statement must repent to his Lord because of this great falsehood he has stated. He must make a clear announcement that they are disbelievers and that they are inhabitants of the Hell-fire. He must also announce that it is obligatory upon them to follow the unlettered Prophet Muhamad (peace be upon him). he is the one that the Jews and the Christians know and they know him like they know their own sons. Allaah has stated,
"Those who follow the Messenger, the illiterate Prophet whom they find written about with them in the Torah and the Gospel, he commands what is right and forbids what is evil. He allows them the good things and forbids for them disdainful things. He releases them from their heavy burdens and from the fetters that were upon them. So those who believe in him, honor him, help him, and follow the light which has been sent down with him, it is they who will be successful." [al-Araf 7:157]
He is the one of whom Jesus, son of Mary (peace be upon him) gave glad tidings. Jesus said, as His Lord states in the Qur`aan,
"O Israel, I am the Messenger of Allaah for you confirming the Torah (which came) before me, and giving glad tidings of a Messenger to come after me, whose name is Ahmad [one of the names of the Prophet Muhammad peace be upon him]. But when he came to them with clear proofs, they said, 'This is clear magic.'" [as-Saff 61:6]
When the one, Ahmad, who was promised came to them with clear signs, they said that it was nothing but magic. By this, we can refute those Christians who say that the one Jesus promised was named Ahmad and not Muhammad. We say to that that Allaah said, "When he came with clear proofs," and none has come to you after Jesus except Muhammad (peace be upon him). Muhammad is Ahmad and Allaah inspired Jesus to call Muhammad Ahmad. This is because Ahmad is a name from the root meaning, "to praise," and the name itself means, "One who praises Allaah more than others." Therefore, [Muhammad (peace be upon him)] is the most praiseworthy of Allaah among all peoples...
I further say: If anyone claims that there is a religion on earth other than Islaam that is acceptable to Allaah, then the person who made that claim is a disbeliever and there is no doubt about his disbelief. This is because Allaah has said in His Book,
"Whoever seeks a religion other than Islaam, it will never be accepted from him and in the Hereafter he will be one of the losers." [ali-Imran 3:85]. Allaah has also said,
"Truly, the religion in the sight of Allaah is Islaam" [ali-Imran 3:19]. Another verse states,
"This day, I have perfected your religion for you, completed My favour upon you, and have chosen for you Islaam as your religion" [al-Maida 5:3]
Based on that - and I repeat it for a third time - the person who made that statement [mentioned in the question] must repent to Allaah, Most High. And he must also make it clear to all the people that those Jews and Christians are disbelievers. This is so because the proof has been established against them, they have received the message but they have refused to believe.
The Jews have been described as those who have earned Allaah's wrath. That is because they knew the truth and chose to go against it. The Christians have been described as those who have gone astray.. This is because they sought the truth but were misled from it. Now, all of them have knowledge of the truth and recognize it. However, they go against it. Therefore, they all deserve to have the wrath of Allaah upon them. I call all those Jews and Christians to believe in Allaah and all of His Messengers. I call upon them to follow the Prophet Muhammad (peace be upon him). This is what they have been ordered to do in their own books. Allaah states in the Qur`aan,
"My Mercy embraces all things. That (Mercy) I shall ordain for those who are God-fearing and give zakat, and those who believe in Our revelations. Those who follow the Messenger, the illiterate Prophet whom they find written about with them in the Torah and the Gospel, he commands what is right and forbids what is evil. He allows them the good things and forbids for them disdainful things. He releases them from their heavy burdens and from the fetters that were upon them. So those who believe in him, honor him, help him, and follow the light which ahs been sent down with him, it is they who will be successful. Say (O Muhammad): O Mankind! Verily, I am sent to all of you as the Messenger of Allaah, to whom belongs the heavens and the earth. There is no true God but He. It is He who gives life and causes death. So believe in Allaah and His Messenger, the unlettered, who believes in Allaah and His Words. And follow him so that you may be guided." [al-Araf 7:156-158].
[By having such faith, those converts from Judaism and Christianity] shall receive their reward twice. The Messenger of Allaah (peace be upon him) said,
"There are three people who will receive their rewards twice: A person from the People of the Book [Jews and Christians] who believed in his prophet and believed in Muhammad..." [al-Bukhari and Muslim].
I have also found in the law book al-Iqna`, in the section on apostasy, where the author has stated, "One who does not consider as a disbeliever one who follows a religion other than Islaam, such as the Christians, or who has doubt about their disbelief or who approves of their way himself is a disbeliever." And he quotes Ibn Taimiya as saying, "[The following is] a disbeliever: Whoever believes that churches are the houses of Allaah in which he He is worshipped or who thinks that what the Jews and Christians do are valid acts of worship of Allaah and obedience to Him and His Messenger, or loves or approves of what they do, or aids them in having such places and establishing their religion, and he does so thinking that is a way of getting closer to Allaah or worshipping Allaah." He also stated, "If someone believes that visiting the Jews and the Christians in their places of worship is an act that brings him closer to Allaah, then he is an apostate."
These statements support what was stated in the text of the answer. This is a matter concerning which there is no room for doubt. And help is sought only with Allaah.
[This question was responded by Ibn Uthaymeen, Fatawa Islaamiya, vol. 1, p. 87] [Excerpted from Abu Lauz, "Answers To Common Questions From New Muslims", translated by Jamaal al-Din Zarabozo. Published by IANA, 1995.]
source:
http://www.islaam.net/main/display.php?id=63&category=3
Allaah Almighty knows best. 
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