Allah SWT is basically saying that if you do the following (coloured font below), you’re on your own! SubhanAllah! It’s like being disowned…how emotionally difficult that is to digest!
Have a look at the verse:
Never will the Jews nor the Christians be pleased with you (O Muhammad Peace be upon him ) till you follow their religion. Say: “Indeed, the Guidance of Allah (i.e. Islamic Monotheism) that is the (only) Guidance. And if you (O Muhammad Peace be upon him ) were to follow their (Jews and Christians) desires after what you have received of Knowledge (i.e. the Quran), then you would have against Allah neither any Wali (protector or guardian) nor any helper. [Suratul Baqarah, ayah 120]
This Ayah carries a stern warning for the Muslim Ummah against imitating the ways and methods of the Jews and Christians, after they have acquired knowledge of the Qur’an and Sunnah, may Allah grant us refuge from this behavior. Although the speech in this Ayah was directed at the Messenger , the ruling of which applies to his entire Ummah.
[Source: Tafsir ibn Kathir]