"Today if you hear his voice, do not harden your hearts..."
It is a rarity among us to hear God's voice directly, so you would think that after hearing from God himself so directly no one would be able to deny or disobey him. Quite the opposite, I'm afraid.
Doubt affects the lives of even the "best" Christians (there is no such thing). Despite every miracle sign and wonder you can witness, it would not be enough to prove so concretely that God is who He says He is. Doubt is inescapable, but the good news is that it is necessary.
You cannot have great faith without great doubt. If you could be so sure than you are either too gullible or your God is too small. Doubt exists to protect us from falling into the snare of lies so readily. To be able to discern truth is a gift from God himself and it is our duty to use this gift wisely. We are creatures of doubt, but God is of faithfulness.
So then do not believe solely on the testimony of another, but taste and see for yourself what is real. Be careful not let your doubts lead you astray, but remember what God has done and can do. Those who believed but turn away are worse off than they started. They fascinate me for their existence has a great implication. Can one lose their salvation, or is it that they never really had it at all?