Indeed, we should strive with clinginess to the statement of Tawḥīd, as it is one of the very important beliefs of the Muslim.
It is important to have knowledge of the nullifiers of Tawḥīd to protect us from going astray from the correct monotheism (i.e, Tawḥīd).
The ten nullifiers are to be listed below, ʾIn shāʾ Allāh:
"And if you ask them, they will surely say, 'We were only conversing and playing.' Say, 'Is it Allāh and His verses and His Messenger that you were mocking?' Make no excuse; you have disbelieved [i.e., rejected faith] after your belief. If We pardon one faction of you — We will punish another faction because they were criminals." (1)
"And who is more unjust than one who invents a lie about Allāh or denies the truth when it has come to him? Is there not in Hell a [sufficient] residence for the disbelievers?" (2)
"And [mention] when We said to the angels, 'Prostrate before Adam'; so they prostrated, except for Iblees (i.e, Shaiṭān). He refused and was arrogant and became of the disbelievers." (3)
"And he entered his garden while he was unjust to himself. He said, 'I do not think that this will perish — ever. And I do not think the Hour will occur. And even if I should be brought back to my Lord, I will surely find better than this as a return.' His companion said to him while he was conversing with him, 'Have you disbelieved in He who created you from dust and then from a sperm-drop and then proportioned you [as] a man? But as for me, He is Allāh, my Lord, and I do not associate with my Lord anyone." (4)
I ask Allāh to allow us to have firm belief in Tawḥīd, stay away from falling into these nullifiers, and make us of the Muttaqūn.
May Allāh bless you, and shukran.
Sources :
Nullifiers taken from the book "Detailed Chapters Clarifying Tawḥīd, Minor Shirk and Major Shirk" with minor changes to the original wording.
1. Qurʾān 9:65-66
2. Qurʾān 29:68
3. Qurʾān 2:64
4. Qurʾān 18:35-38