The first step in a discussion about God is to get an atheist to acknowledge the possibility of any deity existing.
How to Respond
Start by laying out one of the arguments for God’s existence. Show them that it is harder to prove that God doesn’t exist than that He does exist.
Show that the theistic worldview requires less faith than atheism or agnosticism. Atheists must know everything about everything in order to prove that God does not exist. Science, as a tool to measure repeatable, natural phenomenon, cannot measure something that is supernatural. Since God is supernatural, absence of scientific evidence cannot be used as evidence of absence of God.
Agnostics claim that they know for sure that one cannot know anything about God. This is, in it of itself, a logical inconsistency.
Once you have shown that a god could exist, you can then move on to arguing for the Christian worldview and the existence of God.