In the debate over whether Muslims and Christians worship the same God, some people who defend that proposition have cited Acts 17. This is, of course, a locus classicus for natural theology. But the appeal to Acts 17 to prooftext their position raises several issues:
i) I have no objection to natural theology, in the sense of a priori and a posteriori arguments for God's existence. Theistic proofs from reason or empirical evidence.
Sometimes, however, the appeal is circular. There are traditional schools of natural theology, like Thomism and perfect being theology (Anselm). Some people come to Acts 17 with that package in mind, and interpret Acts 17 in light of that package. In their case, they aren't deriving their concept of natural theology from Acts 17; rather, they filter Acts 17 through their preconception of natural theology.
They act as though Paul is a Thomist. They assume Paul is very straight-laced in his dialogue with the Athenians. That he has a positive view of their piety. Although their piety is deficient, it is true so far as it goes. They begin with the God of natural revelation. It simply needs to be supplemented.
ii) That, however, is a very anachronistic and naive reading of the text. To begin with, there's more than one audience for this address. There is Paul's audience, and then there's Luke's audience. Paul's audience are pagan philosophers or dilettantes, whereas Luke's audience are Christians. Moreover, Luke expects his readers to understand OT theism. That's the standard of comparison.
In addition, this pericope opens with the programmatic statement about Paul's stern disapproval of Athenian idolatry. That sets the tone for the rest of the presentation. That's something the reader is privy to, but not the Athenians.
So the discourse reads at two different levels. For Luke's audience, there's a running irony in Paul's statements that would be lost on his pagan listeners.
iii) This is reflected in Paul's backhanded compliment to their religiosity. Paul uses an ambiguous word (deisidaimon/deisidaimonia) that has both positive ("pious") and negative ("superstitious") connotations. A double entendre that would mean one thing to Paul, but something else to his audience.
Some commentators reject the negative connotation because they think that would be off-putting to Paul's audience, but that misses the point. English has no word with the same ambiguity, but Paul didn't have to choose between a flattering word or a pejorative word. His audience wouldn't catch on.
This allows him to preserve a certain distance. Common ground without complicity.
iv) Along with his appeal to an altar to the "unknown god," this is part of Paul's captatio benevolentiae, in which a speaker curries favor with the audience to gain a hearing.
In context, the "unknown god" is not the monotheistic Creator. Rather, erecting an altar to an unknown god is a way for fearful pagans to cover their bets. There are many gods they never heard of. Nameless gods who might be offended if there was no altar in their honor. You didn't want to get on the wrong side of a god or godless, so this is placeholder for all the other heathen deities the Athenians haven't heard of.
Paul cleverly exploits this as a bridge. A tongue-in-cheek way of making a serious point.
v) V28 contains one or possibly two quotes from pagan authors. Some commentators think the line about how we "live and move and have our being" in God is an allusion to Epimenides, but that's disputed. If it's from Epimedides, the god to whom that's originally addressed is Zeus. In any event, that's certainly the case in reference to the quote from Aratus.
That, however, poses a dilemma for people like Francis Beckwith who say Muslims and Christians worship the same God. For they restrict that to the "Abrahamic religions" (Islam, Judaism, Christianity) in contrast to polytheism. Yet the context of Aratus is not to the monotheistic Creator, but to Zeus. So this either proves too much or too little for people like Beckwith.
An exception would be Michael Rea, who goes so far as to say:
Christians and Muslims have very different beliefs about God; but they agree on this much: there is exactly one God. This common point of agreement is logically equivalent to thesis that all Gods are the same God. In other words, everyone who worships a God worships the same God, no matter how different their views about God might be.
That, however, cuts agains the grain of Acts 17, which, among other things, continues the OT polemic against pagan idolatry.
Paul simply disregards the original setting because he's using this passage as a pretext to smuggle in a witness to the God of OT revelation, culminating in the revelation of Christ.
Perhaps some people resist the ironic reading because they they imagine it would be unethical for Paul to resort to duplicate–as they view it. There is, however, nothing unethical about irony. Moreover, Paul believes what he says. But he's using certain tactics as a point of entree. That's not a foundation to build on, but a way of making people listen to something that will replace it.