Atheism, as it is usually defined is a “lack of belief”. It therefore follows that it is also a lack of knowledge. To “know something” in philosophy or to have “knowledge of” an object, in this case God, from an epistemological foundation, is to have a “justified, true, belief.”
One can deduce that Atheists do not have “knowledge” of God which implies that they lack justification and validation, since the only lacking implied in the definition of “atheism” is belief.
So they lack knowledge about God. The goal of the theist then logically would be to provide them with a “justified, true, belief”, or in other words, knowledge.
So in response to the question posed in this thread, “How do I convince an Atheist that God exists?” The answer: educate them.
Atheism is merely ignorance. The challenge for the theist is to not appear condescending, or injure the person’s pride, when attempting to educate someone about an obvious truth.
Keep in mind that Catholics have much more intellectual ammunition at their disposal. 2,000 years of thought. As opposed to “Atheism” which only really has since the enlightenment, maybe 300 years. We also have the gregorian calendar going for us. Which is universally accepted. The world has unknowingly decided to use the birth of Jesus Christ as a measure of the beginning of time.