In Acts 17:16 we are told that Paul was grieved because the whole city of Athens was given over to idolatry. So, in vv. 22–28, he tells his hearers that they had other gods before the God of creation. He is in essence opening up the First and Second Commandments: “I am the Lord your God…You shall have no other gods before Me. You shall not make for yourself a carved image” (Exod. 20:2–4). Then, in vv. 29–31, he rebukes them for their idolatry and preaches repentance and future punishment by the Law (God’s standard of “righteousness”). So I wouldn’t say that he began with creation as opposed to the Law. It was what he used to point them to the Law.
Think of how Jesus approached the woman at the well (in John 4). He began to speak to her about water, but then He spoke to her about her violation of the Seventh Commandment. When Nathan was commissioned by God to reprove David for his sins, Nathan began in the natural realm, and then pointed out David’s transgression: “Why have you despised the commandment of the LORD?” (2 Sam. 12:9).
Although Paul mentioned creation, he didn’t stay there for long, because speaking about creation doesn’t convict a man of his sins. There is no guilty conscience accusing hearers as long as we speak apologetically. The goal is to use the Law to bring the knowledge of sin (see Rom. 3:20).
Be careful not to get caught up in a sword fight about evolution or atheism. These subjects should merely be seen as a means to an end. The end is the preaching of the reality of Judgment Day and the terror of hell—the cross, repentance, and faith—and the biblical way to get there is through the Law.
Acts 17:16 Now while Paul waited for them at Athens, this spirit was provoked within him when he saw that the city was given over to idols.
Ray Comfort's Daily Evidence 
It is important for us to tell Christ to other and bringing them to the Law. The Law convicts and brings people to brokenness and repentance. It doesn’t make any sense to preach a Gospel of repentance when “there is nothing to repent with” in the first place.
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I agree. And the biggest challenge for many preachers is ‘the swing from the natural to the spiritual’ – or talking from the Word in general, to speaking from (not of) the Law in particular. This is challenging. I could easily make some reference to the laws of God, His standards etc., but to read from the Law, or ‘open up the Law’ can even cause ‘Christians’ to cringe!