I would ultimately tell them that, regardless of their life views on evolution, the earth's age, origin of the universe, etc, they will NEVER fully know God by simple reasoning alone. No matter how many books they read and how many second opinions they get, eventually there will be a leap of faith that they have to make.
For example, say there's a person you are interested in getting to know. You can ask questions about them from other people, Google their name and look them up their Facebook, you can simply observe them, watch how they interact with other people, etc. But eventually, you have to put any pre-suppositons, biases and misconceptions aside and simply go and meet them face to face, to see who they really are. Eventually this person will speak to you, and - if you are truly listening - will reveal things to you that you would have never known by simply observing them from afar. And once this person has revealed to you who they really are, you come to the crucial question: do I trust them?
So many people today approach Christianity with the mindset of "I will ONLY put my faith in what I can absolutely verify and empirically know as proven fact." But that's not faith, that's knowledge. This is actually hypocritical and ironic because there are SO many things in our lives that we DO take on sheer faith.
For example, why did you marry the person you married? Was it because you had carefully compiled all the data and evidence about him/her and thus reached the statistically-proven conclusion that he/she will be exactly the kind of spouse you want? I certainly hope not. For any healthy marriage, the basis of it will be faith and trust in one another. You didn't know absolutely everything about your spouse when you said "I do", but you knew enough to trust them. If your marriage is active and healthy, you will learn more about them each and every day.
I've always liked Psalm 34:8: "Taste and see that the LORD is good; blessed is the one who takes refuge in him."
"Taste and see" does not mean, "Check out God's credentials" (although they ARE impressive). Instead it is a warm invitation: "Try this, I know you'll like it." When we take the first step of obedience in following God, we cannot help discovering that He is good and kind. When we begin the Christian life, our knowledge of God is partial and incomplete. As we trust Him daily, we experience how good He is.
So if people want solid evidence and rational reasons for choosing Christianity, well certainly they ARE there, and there are many. But while they can certainly aid and enrich our faith, they can never substitute faith itself, which ultimately comes down to trust.