Allah revealed in Surah Al-Fath (S48) Ayah 13:
وَمَنْ لَّمْ يُؤْمِنْ بِاللَّهِ وَرَسُولِهِ فَإِنَّا أَعْتَدْنَا لِلْكَافِرِينَ سَعِيرًا
which means: Whoever does not believe in Allah and His Messenger, then certainly We (meaning Allah) have prepared for the kafirs an inferno. [A sa^ir is a fire which has flames upon flames].
Also Allah revealed [Surah Muhammad (S47) Ayah 34]:
إِنَّ ٱلَّذِينَ كَفَرُوا وَصَدُّوا عَن سَبِيلِ ٱللَّهِ ثُمَّ مَاتُوا وَهُمْ كُفَّارٌ فَلَن يَغْفِرَ ٱللَّهُ لَهُمْ
[which means] “Surely, Allah will not forgive those who blasphemed and those who blocked others from the Path of Allah, and then died while being blasphemers.”
The Islamic scholars have agreed on charging with blasphemy whoever practices a religion other than ‘Islam, as well as charging as a blasphemer whoever does not charge him with blasphemy, or doubts, or remains neutral, like for one to say, “I do not say he is a blasphemer and I do not say he is not a blasphemer.” Al-Qadi ^Iyad in his book Ash-Shifa stated that there is a consensus that the one who adopts a religion other than Islam is kafir.
The true religion is ‘Islam. The Exalted said [Surah Al ^Imran (S3) Ayah 85]:
وَمَن يَبْتَغِ غَيْرَ ٱلْإِسْلَٰمِ دِينًا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِى ٱلْـَٔاخِرَةِ مِنَ ٱلْخَٰسِرِينَ
[which means] “Whoever seeks other than ‘Islam as a religion, it will not be accepted from him, and he shall be, in the Hereafter, among the losers.”