The Explanation of the Treatise: The Four Principles of Shirk

So when the believer becomes aware that when Shirk is mixed with Tawheed, it becomes corrupted just like when al-hadath corrupts purification; then he should become aware that it is necessary to learn Tawheed and Shirk as it really is so he would not fall into Shirk.

Because Tawheed by definition is the religion of Allah, it is Islam, it is the guidance. So when the person does any type of Shirk this nullifies his Islam, this nullifies His religion.

For example, he supplicates to the deceased seeking the aid of them. He curses/insults the religion or Allah or His messenger, making mockery of Allah or His messenger or the religion; believing making permissible what Allah had made impermissible from what is known from the religion out of necessity, like committing fornication and the likes. Thus when he carries out any one of the nullifiers of Islam then he has nullified his religion.