Um that is a very hard question for me, since my family raised me in a very Christian home. I see so many things that make me believe he isn't, but then there are things that I can't explain. I say for me, its a 20-30%.
I am 100%sure and for certain God is real and Jesus died on the cross for my sins; arose on the third day: and by his grace I will be with Him in the air: 1 Thessalonians 4:16-18 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the air, and so shall we ever be with the Lord. Wherefore comfort one another with these words. If you want to get to heaven: you had better be in 100% or I assure you it will not happen; its all or nothing think about it and make a choice, and make the correct choice.
I'm not putting some silly arbitrary percentage on it, but if I did, it'd be dreadfully low. Lower than Bigfoot, ghosts, and the Loch Ness Monster. Then again, you're going to have to define your god. I could be a called a theist to some.
I am 100% sure that God is absolutely real. I'll admit, I grew up in a Christian home, went to church every Sunday and Wednesday, and read my Bible, but I still had my doubts. But you know what? God revealed Himself to me through His Word, through logic, and faith completed my belief in a God who loves us and wants to save us. http://www.icr.org/god/. Here's a few evidences for God. And remember Psalm 14:1 and 53:1, "The fool hath said in his heart, there is no God."
I'd like to say that I am 100% sure, but I don't think I really am. I don't think I would do what's right and continue to say I believed in Him if my life was threatened. That's why I am so glad that Christ said He would be the one to die on a cross; because I wouldn't be able to do it.
100percent it has been built in me from birth the common sense God is real, When a plane is heading straight to the ground everyone on the plane calls out "Oh God help me" showing the truth of what they believe. You people are only saying you don't believe because you live miserable and sad lives and so you blame God instead of blaming your own intellect for not understanding the purpose of the trial your going through. This life is a 2:1 relation of ease and hardship whether you blame God or not this will not change untill you enter hell or paradise.