This quote comes from the third chapter of this novel, and is used to express the intense guilt that Stephen feels after engaging in his period of debauchery. This quote uses a very strong simile to explore how he feels about his sin, as Stephen views it, comparing his soul to "a beast in its lair." Now, however, as he hears the preacher that he listens to, he imagines the "blasts of the angel's trumpet" moving him from "the darkness of sin into the light." Note how the text describes Stephen's feelings as he listens to the preacher's words just before this quote appears in the text:
Every word of it was for him. Against his sin, foul and secret, the whole wrath of God was aimed. The preacher's knife had probed deeply into his disclosed conscience and he felt now that his soul was festering in sin. Yes, the preacher was right. God's turn had come.
Note the use of the short sentences that add impact to Stephen's feelings and how he views the preacher's words as being particularly meant for him. It is interesting how he views the words of the preacher as his "knife" that cuts into him, whereas actually it is clearly his own conscience that is being aroused by the preacher's words after months of being sidelined and ignored by Stephen as he embarked upon his period of debauchery. It is clear though that this quote signifies a transition point in Stephen's character, as he moves away from unlicensed hedonism towards a much more structured existence where religion is placed at the centre of his identity.