Surah Al-Baqarah — Ayah 120 (2:120)
وَلَن تَرۡضَىٰ عَنكَ ٱلۡيَهُودُ وَلَا ٱلنَّصَٰرَىٰ حَتَّىٰ تَتَّبِعَ مِلَّتَهُمۡۗ قُلۡ إِنَّ هُدَى ٱللَّهِ هُوَ ٱلۡهُدَىٰۗ وَلَئِنِ ٱتَّبَعۡتَ أَهۡوَآءَهُم بَعۡدَ ٱلَّذِي جَآءَكَ مِنَ ٱلۡعِلۡمِ مَا لَكَ مِنَ ٱللَّهِ مِن وَلِيّٖ وَلَا نَصِيرٍ ١٢٠
English translation (Saheeh International)
And never will the Jews and the Christians approve of you until you follow their religion. Say, "Indeed, the guidance of Allāh is the [only] guidance." If you were to follow their desires after what has come to you of knowledge, you would have against Allāh no protector or helper.
Brief commentary
Allah addresses and warns His Prophet, telling him that the Jews and the Christians will never be happy with him unless he leaves Islam and follows what they do; and that if he, or one of his followers, were to do such a thing, after the clear truth has come to them, they would not find protection or help from Allah. This makes clear the seriousness of leaving the truth and of following people of falsehood.