If you don't believe that there's a God, it doesn't mean he doesn't exist, it just means that you've turned your back against Him and you're walking towards darkness. He knows exactly why you've lost faith in Him because He's "all knowing", but he'd really want you to make the effort to believe in Him and come out of darkness into His arms because not only is He "all knowing", but He's merciful as well.
God does exist and I believe in signs. If you've seen the movie "Fools Rush In" you'd understand what I mean about "signs". If you don't believe in God, just go out of your way and take a little chance to say a little prayer at night, ask Him to give you a sign that He exists. God may not give you a sign right away, but when He does, it could be in your dreams or from things that you notice when you're awake. But the problem is, Satan can gives signs as well because he's a trickster and can take many forms, and so in that case you'd really have to use your heart and your mind to distinguish whether the signs are really coming from God or Satan himself. And, if you believe a sign is coming from God, Satan is of course relentless and would try to make you believe that the sign isn't coming from God and is purely coincidence.
As a Catholic, I have learned that if you start to feel tempted by something, whether it's something that has to do with greed, anger, lust, laziness or some other kind of sin, just think about how Jesus died on the cross for all our sins, the sharp terrible pain that He went through being nailed on the cross and the following unimagineable excruciating pain He endured hanging on there for dear life. Certainly what dear Jesus went through would make you feel guilt about having that temptation in your mind in the first place and make it go away. And if you start to feel that same temptation over again an hour later or whatever, just keep thinking the same thing about how Jesus died for our salvation. God wants us to feel sorry for our sins.
And believe me, God isn't the one responsible for the bad things that happen in this world, it's the work of man because since the beginning of time God gave man "free will", or in other words the power to make the free decisions in this world.
God really does exist and the longer you stray away from Him, the more sins you will commit and the more you will hurt other people. There's this saying that goes something like: "I didn't believe in God until He was all I had left" and it holds true because take someone for example who doesn't believe in God and he feels nothing is holding him back and so he does less serious things like stealing or dealing drugs but then it would esculate to painfully torturing innocent people and killing them and he ends up on Death Row and isn't it interesting how Death Row inmates who never believe in God in their lives always requests for a priest to to talk to right before they're put to death.