It is a must that one should believe in the oneness of Allāh and not associate any partners with Him, which is polytheism, a sin that is never forgiven (unless one repents from it before death).
In Allāh's (سبحانه و تعالى) book, there contains numerous Āyat that affirms His oneness. The Āyat are to be listed below, ʾIn shāʾ Allāh.
Allāh (سبحانه و تعالى) says in His book:
"And whoever invokes (or worships) besides Allāh another deity for which he has no proof — then his account is only with his Lord. Indeed, the disbelievers will not succeed." (1)
Allāh (سبحانه و تعالى) says in His book:
"And [mention, O Muḥammad], when Luqmān said to his son while he was instructing him, 'O my son, do not associate [anything] with Allāh. Indeed, association [with Him] is great injustice.'" (2)
Allāh (سبحانه و تعالى) says in His book:
"And [He revealed] that the Masājid (i.e., the term (masājid) here includes every place of worship or the earth in general) are for Allāh, so do not invoke (or worship) with Allāh anyone." (3)
Allāh (سبحانه و تعالى) says in His book:
"To Him [alone] is the supplication of truth. And those they call upon besides Him do not respond to them with a thing, except as one who stretches his hands toward water [from afar, calling it] to reach his mouth, but it will not reach it [thus] (i.e, the analogy indicates that false deities will never respond to them at all). And the supplication of the disbelievers is not but in error [i.e., futility]." (4)
Allāh (سبحانه و تعالى) says in His book:
"That is because Allāh is the True Reality, and that which they call upon other than Him is falsehood, and because Allāh is the Most High, the Grand." (5)
I ask Allāh to keep us sincere in the religion of Islām, and allow us to act upon these glorious Āyat of al-Qurʾān.
May Allāh bless you, and shukran.
Sources :
1. Qurʾān 22:117
2. Qurʾān 31:13
3. Qurʾān 72:18
4. Qurʾān 13:14
5. Qurʾān 22:62