Tawhid is the basis of religion

Sheikh Muhammad ibn Suleiman (may Allah have mercy on him) said:

“ Know that tawhid in worship is that for which Allah created creation, that for which He sent down the scriptures, that is why He sent messengers, and this is the basis of religion, without which there will be no Islam is correct, and the one who leaves it and gives Allah a partner will not be forgiven, as the Almighty said: “Indeed, Allah does not forgive when a partner is given to him and forgives what He wishes less than this”, (Nisa 48)” .

See “Ad-durar As-saniya”, 1/138.