Actually, to be saved, you must believe in the sacrifice of Jesus(God-in-the-Flesh), and if you believe in that then you obviously believe in God. Remember John 3:16: "For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. " So you must believe that Jesus died for you and that He can/has forgiven you of your sin. For the other part of your post, the doubt part, that depends on what view you believe in; the Arminian view or the Once Saved Always Saved view. In the OSAS, if you fall away and disbelieve later on in your life; you either never _really_ believed, or you are still saved. In the Arminian view, if you fall away, or backslide, then you must believe in God/the sacrifice/His power & ability to forgive you of your sins, sometime before you die, and repent of your disbelief; if you don't believe again though, atleast from the Arminian viewpoint, you will be eternally seperated from God after you die. So you may have some doubts sometime, but you should really believe that God came done and died for you and that he has forgiven you of your sins, at the very least, you must believe this before you die. I don't really see how you can not believe in God, just look at the age and validity of the Bible, the beauty in the world, the size/beauty/non-randomness of the universe, how much smarter/different humans are compared to animals. Also, their is _much_ reason to believe in Jesus being God in the flesh and dying for humanity; for one, it's a fact that Jesus walked the earth, look at the prophecies in the OT concerning Jesus' coming. If he wasn't God; why would all of His followers not just lie about worshiping Him, and why would they not just deny the resurrection; that way they would not have had to be be-headed, tortured, etc. The fact is; no one deserves to be going to heaven; we have _all_ sinned and fell short of God's perfection, we have all broken His laws, we cannot be in front/near Him in heaven; but Jesus came down to die for us in order for us to be forgiven of our sins, so that we could be without blame in God's eyes. You cannot do anything to get your way into heaven, God has already done _all_ of the work; you just have to believe in Him and what he did.
Edit: Misspelt a word.