Tip #2 - Establish a point of contact
If sharing the gospel message is point B, Paul starts by acknowledging their point A. Notice that he does not insult their beliefs, but he does not affirm them either. Instead, he establishes a point of contact, a sense of commonality, so he can build a bridge.
In today's world, it would be hard to find a true atheist. Most people are spiritual in some way. Their spirituality may stem from a belief in "The Universe", an "unknown god or gods", "themselves", or the ideas that "everything is god" or "god is in everything".