OP. So sorry for your situation. I can imagine it is incredibly hard. But I can only imagine. As it is so far from "normal".
Please take that on board. This is a very unhealthy environment for you and your children in so many ways. If you had met him and he was like this, you wouldn't have dated him. You wouldn't have wished to raise children with him. But it has slowly become your normality, which is why this is so hard.
What is frightening is that it is becoming your children's normality. Your eldest even is far too young to be hearing his very extreme views without any form of balance. If they continue to be subjected to it, it will help form their thoughts and their normality, in the same way it has yours.
You say you don't know how many more chances to give him. This isn't about chances. He isn't cheating on you and making conscious decisions which he has a choice over. He is suffering with extreme mental health issues. Part of that issue is an absolute conviction that he is right and the majority of the world is wrong. When he is better for a bit he is simply stifling his extreme views to keep you on side. They are still there because he still has not had any treatment. And he cant see that he needs it.
You either need help sectioning him so he can receive treatment it is so abundantly clear he needs or to part so you are not at risk.
And I appreciate you say this:
I have not seen them, and hand on heart do not believe he would hurt my children. He is besotted with them
But mental health can disintegrate or have momentary lapses in an absolute heartbeat. And you would simply never forgive yourself if in a moment he did do something.
I imagine six months into your relationship if someone had told you he would end up like this, you would have hand on heart said no way at that time.
As I say, I can only imagine how tough it is. But you are so at risk, as are your children. Mental ill health is not rational, it does not sometimes remember who the person loves. Please, keep you and your children safe.