"A spiritual application of the Word of God consists in applying it to the heart, in spiritually enlightening, sanctifying influences. A spiritual application of an invitation or offer of the gospel consists in giving the soul a spiritual sense or relish of the holy and divine blessings offered, and the sweet and wonderful grace of the Offerer in making so gracious an offer, and of his holy excellency and faithfulness to fulfill what he offers, and his glorious sufficiency for it, so leading and drawing forth the heart to embrace the offer and thus giving the man evidence of his title to the thing offered."
Jonathan Edwards, The Religious Affections, page 153.
This is what the Holy Spirit is doing to the hearts of people as they hear the gospel preached. It is quiet and invisible, uncaused and unstoppable, for God's glory.
Have a great Sunday.