In Theism there is evidence.
Christianity is not a philosophy or a belief system without historical roots. Everything that a person is asked to believe about Jesus Christ is founded upon the evidences of witnesses. The Bible never asks a person to "Just have faith" without evidence to back it up. Jesus never asked anyone to believe in him without evidence. Evidence gives belief in Jesus its strength.
In Theism there is purpose. Everyone deserves purpose. It is through purpose that we find meaning in our lives. If Christianity is backed up by evidence, then it implies that there is a real purpose to its claims. We don't believe simply to believe or to make ourselves feel good. We believe because the evidence tells us Jesus is true. And if Jesus is true, then he has a purpose for our lives. And if he has a purpose for our lives, then we can have meaning.
In Theism there is meaning.
God didn't simply create us like marionettes and cut the strings. God has a plan and purpose for our lives. He wants us to encounter him and through that encounter to discover meaning. We all need something of significance. We often create our own meaning by creating our own purpose. But wouldn't a purpose given to us by God give us lasting significance?
In Theism there is love.
Love is not unknown to other religions, or even to the atheist. But, "No greater love has anyone than he lay down his life for his friends." Jesus said that. But he went further. For in the Bible we read that while we were still God's enemies, Christ died for us. In Jesus there is love greater than what we can experience without him.
Why should an atheist consider theism? Because theism offers everything that atheism cannot. Because theism offers truth. Because theism provides a way to deal with our sins and failures. Because theism redeems and doesn't leave us to deal with life alone.