Almachius is a classic an antagonist. Cecilia wants to live a Christian life, and Almachius exists to thwart that purpose. Part of his role in the tale is to serve as an obstacle against which Cecilia can demonstrate her sanctity. He gives Cecilia the ultimatum – worship Jupiter or die – that enables her martyrdom. So actually, in a strange paradox, Almachius ends up fulfilling Cecilia's goal in the end. But, since that really wasn't his intention and was due largely to God, he's still our antagonist.