Introduction to Islam
"If anyone desires a religion other than Islam (Submission to Allah), Never will it be accepted of him; and in the Hereafter he will be in the ranks of those who have lost."(Qur'an 3:85)
"This day have I perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion". (Soorah Al-Maa'idah 5:3)
"Abraham was not a Jew nor Christian; but an upright Muslim." (Soorah Aal'imraan 3:67)
"And behold Allah will say: "O Jesus the son of Mary Did you say to men, Worship me and my mother as gods besides Allah He will say-"Glory to you I could never say what I had no right (to say')" (Soorah Al-Maa'idah- 5:116)
If a person does not pronounce the Shahada and enter Islaam, we cannot say that he is a Muslim, even if he admired Islaam and recognized that it the best religion, a great religion, and so on. Abu Taalib, the uncle of the Prophet (peace and blessings of Allaah be upon him) died a kaafir (non-Muslim), and Allaah forbade His Prophet to pray for forgiveness for him, even though he used to defend the Prophet (peace and blessings of Allaah be upon him) and said in his poems: "I do know that the religion of Muhammad is better than all other religions of mankind; were it not for fear of blame or slander, you would find me accepting it completely." (source islamworld.net)
That which you have been given is but a fleeting comfort of this life. Better and more enduring is that which Allah has for those who believe and put their trust in Him; who avoid gross sins and indecencies and, when angered, are willing to forgive; who obey their Lord, attend to their prayers, and conduct their affairs by mutual consent; who bestow in alms a part of that which We have given them and, when oppressed, seek to redress their wrongs. [Al-Qur'an, Sura: 42 (Ash-Shura), Ayat: 36-39]
"For We assuredly sent amongst every people a prophet,(with the command) worship meand avoid false gods " (Soorsh Al-Nahl 16:36)
"I have only created Jlnns and men, that they may worship me"(Soorah Zaareeyaat 51:56)
"Verily Allah will not forgive the joining of partners with Him, but He may forgive (sins) less than that for whom so ever He wishes"(Soorah An- Nisaa 4:48 and 116)
If only they had stood by the law, the Gospel, and all the revelation that was sent to them from their Lord, they would have enjoyed happiness from every side. There Is from among them a party on the right course; but many of them follow a course that Is evil.". (Soorah Al-.Maa'idah 5:66)