Scripture says even the Demons believe and know God.
Quote that one please. Demons believe God is real, but they don't "believe in Him" as Christians do. Demons know God? Like His children do? I don't think so.
John 17:3 KJV
And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Also, Scripture says to work out your salvation with fear and trembling. How do you reconcile that with having no doubt?
Do you see some conflict in this? Are you thinking God intends that you spend your live afraid that you are not really saved? If this is how you feel I suggest you get this straight with God. Maybe you don't truly know Him? I don't know, I can't say for you.
You can be afraid of not being saved, and you can be afraid of falling short of His intentions for you.
1 John 3:1-3 KJV
1) Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2) Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but
we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3) And every man that hath this hope in him purifieth himself, even as he is pure.
There is nothing wrong with certainty. I know that I'm married to my wife, and I know I'm regenerated in Christ.
If you are not certain, I suggest you make certain before it's too late.
Much love!