Many people who simply enjoy listening to popular music do not realize that it has been used to express religious and political messages.
Conclusion
Popular music has been used to express religious and political messages.
Intermediate Conclusion (After all)
Popular music has repeatedly been adopted by social movements to express their viewpoints.
Evidence (since)
Popular music has the potential to contribute to the "conversion" of nonmembers to the movement's position, as well as to raise the morale and to express the solidarity of the movement's participants.
Evaluate
Most arguments on Main Conclusion and Role are Upside Down Arguments (the conclusion comes before the evidence).
On Main Conclusion, 65% of them have a Pivot / Rebuttal conclusion 25% of them have a First Sentence conclusion
The first sentence has some slight rebuttal vibes because the author is saying something that many people don't realize. It otherwise doesn't sound super opinionated, so it's not an obvious contender for being our Main Conclusion.
But then, "after all" tells us that what follows is support for the sentence before it, so the first sentence is the main conclusion and the "after all" is support. We then see that the "after all" claim is supported by a "since" supporting idea. This tells us that the "after all" claim would count as an intermediate conclusion. It's supporting the main conclusion in the first sentence, but it's being supported by the third claim in the stimulus.
Goal
Look for the closest match for the first sentence. We can lop off the part about "many people don't realize that X" and just look for X. The author didn't try to convince us that many people don't realize something. He just tried to convince us of the idea they don't realize: pop music has been used to express religious and political messages.