The probability of God is 100% for most of us who have had encounters with him and have been born of his spirit. To others, the probability is not likely to be static, that is it varies with individuals. In my 58 years as a Christian, born/baptized by his spirit I've become as convinced of the god Jehovah, as well as his son, spirit, and book that with me it's as 100% as it is that my wife exists.
His reality is such to me that I have a reverential love/fear relationship with him as his child, much like that which an earthly child should have to his/her earthly father. My earthly father loved me and I loved him, but that doesn't mean I had no fear of disobedience to him. Same with God.