Summary: Paul started off his message by describing the altar built to the "Unknown God' whom the Greeks acknowledged and worshipped, then tells them about the true and living God and how to know Him personally.
Paul has established common ground with the men of Athens by pointing out the empty altar dedicated to the God whom they know nothing of, yet honor in their personal ignorance and uncertainty. Paul does not chide them or charge them with blasphemy, nor does he rebuke them for their polytheism. He commends them for taking the time and trouble to set up the altar as a precaution against possibly angering this unknown deity and bringing a curse or plague or other distressing event upon them. They're covering their ground, so to speak. It is as if the apostle were telling them, "Gentlemen, I admire you for your dedication to your religious heritage and for making sure you don't overlook any deity, even if you know nothing about him. You are so close. Now let me have your further attention as I tell you about this God and fill in the details for you. He is so amazing and wants you to know Him. Ready?" Now, look at this encounter from the point of view of these curious Greek leaders. They may be whispering to themselves, "Hey, this guy knows something about us. He's obviously interested in us and our culture. This might be interesting.", and so they pay attention as to what Paul will say next.
Paul is using the same method as the Lord Jesus did in winning souls. Jesus lived and worked among the people, getting to know them, talking with them, playing with the children who obviously loved to be around Him, and getting a huge kick out of frustrating the pious and religiously uptight Pharisees who tended to look down on the people with contempt, labelling them "sinners". They would not talk to or even acknowledge someone like a prostitute or tax collector or other socially stigmatized group or individual, such as the Samaritans. I never read of any child coming up to a Pharisee for a hug or to hear a story, nor of anybody approaching them for advice or prayer. Read the story of the Pharisee and the Tax Collector (Luke 18:9-14) and you see not just their arrogance, but a self-exalting attitude towards God along with the disdain they had for the commoner who approached God for mercy and grace. Your read through the entirety of the Gospels, and you see the Lord Jesus was always available, approachable, and willing to take the time to be a listening Friend who does stick closer than a brother.
All this opened the way for Jesus to tell them about the living God who loved them, yet held them accountable for their lives and to show that we are all sinners in need of a Savior. He provided that by His sacrifice for our sins on the cross and by HIs resurrection from the dead. Jesus showed that He loves us, cares for us, and has been among us, knowing both suffering and joy. His words and work provide us the one way to peace with God (John 14:6) and eternal life in heaven free from sins, sickness, and the evil of this world. He will give us a new heaven and a new Eden forever (Revelation, Chapters 21, 22). It is the Lord Jesus who wipes away tears, puts His loving arms around the grieving, uplifts the soul, and provides mercy, grace, joy, and salvation that will never be taken away from us (John 10:28-30; Romans 8:31-39; Hebrews 13:5).
Paul took his cue from his Lord and Master. He was approachable and open with everyone whom he met and was always ready to share Jesus with anyone anywhere. The Athenians were no exception as he proceeded to tell them about God's role as the Creator and Sustainer of all things seen and unseen. All things came into existence by His Word, and nothing exists that He did not first put into place (John 1:1-4; Colossians 1:`16-18). All things operate according to His will and direction and He is not confined to the columns and interiors of temples, the pedestals of altars, or the imaginations of the writers and storytellers who composed mythical tales of heroes and legends of ancient civilizations. He is unlimited, beyond the scope of time, space, and matter, operating outside of it and sees the past, present, and the future of the world and the universe and has their times and seasons set in a specific mode of operation, precision, logical operation, and rational function all working with a purpose and goal.
With Him, there is nothing accidental, insignificant, overlooked, bypassed, ignored, forgotten, or cast aside as cosmic rubbish and extras. Under HIs rule, all events, persons, the laws of nature, the progression of history, the rise and fall of tyrants, nations, empires, and the providential placement of both the famous and oblivious within specified times of life and achievements, both for good and bad are put into operation. What we believe to be tragedies and travesty is limited to what we see and think. We are not privy to the total and complete picture of what and why everything occurs except that all of creation is under the control and direction of God for reasons that we might not know this side of eternity. There are things which belong to the LORD alone and we are not meant to understand all of it. This is why He has given us the concept of faith and trust. He put everything into motion and He knows how it will all turn out, so let Him carry on and just be thankful that He has not abandoned us to merely exist and then end up in hell for our sins and rebellious attitude towards Him. Be thankful that He DOES let us in on some plans and will reveal things to us when we are ready. Be thankful that He has provided salvation, mercy, love, and grace to us by the saving work of Jesus Christ, even though we do not deserve it. He has provided a home for us with Him, and He will return to take us there (John 14:1-3; 1 Thessalonians 4:13-18). This is the great and mighty God whom we know through Scripture and by the mercy He has shown us throughout our lives. This is the God whom Paul introduces to the spiritually starved Athenians, and he will elaborate on this further as the message continues in Acts 17:26-34.