1. Most die-hard Atheists are smart (although we believe they are unwise) and know why they are an Atheist, just like you hopefully know why you are a Christian.
2. Christians do not believe that ancient gods of other religions are total figments of the imagination. Rather we believe that it was man reaching for a God that did and does exist and doing their best to understand him and how he governs his creation. So when an Ancient Greek prayed to Zeus, he was praying to a version of God that was partly his invention, and partly a remaining knowledge of the Living God. For after the fall, God did not leave man entirely. No, man still could sense God's presence but could not truly know God without God Himself making Himself fully known, as he did with the Jews and then fully in Christ.
3. God is not a God of the Gaps. He is the God of everything, including the Gaps. We believe he is active in some way, without destroying the innate free will, in every created thing, for He holds together all things by his thought. This may not convince, but it will keep you from trying to find the thing that cannot be explained by science. At the core of EVERYTHING created there is a mystery. Period.
4. Believing in God is not the same thing as believing in a pink burping spaghetti monster. Just like believing that my mother loves me is not the same thing as believing that my phone loves me. It is common for Atheists to build arguments built on “Straw Men.” They can no more prove scientifically that God does not exist than we can prove scientifically that He does exist. I can prove that the pink burping spaghetti monster does not exist. I made him up. I defy any atheist to locate the man that made up God.
5. They will rarely be convinced by preaching or your words but by your love. YOU, YOURSELF will NEVER convert them. Only God can draw a man to Himself.