Plato
Not only is Plato important for understanding philosophy, he is also important for understanding Christianity. Not that Christianity is based on Platonism but it has been affected by it, both in unorthodox ways through the Gnostics and orthodox ways through Augustine.
Unless you have the time to study philosophy, you need some sort of quick reference to Plato. I have found Bernard Williams‘ Plato to be helpful. This is a little book in that it can be read in one sitting. Even though it is short, it represents solid scholarship. It will give you the snapshot you need. The great thing about it is that you can get for only a penny (plus shipping). I have read it three times and it is definitely worth more than that.
