what is the Holy Spirit’s gift or mission if not that of infusing due love? The Holy Spirit, then, is given by God to man when due love residing in the divinity is inspired into the human soul. In fact, when this Spirit enters the rational soul, he inflames its sentiments with divine ardour and transforms it by communicating to it a character similar to his own, in order [to enable] it to express back to its own Creator the love it owes him.
Actually, what is the Holy Spirit if not a divine fire? After all, every love is a fire, though a spiritual fire. That which material fire does with iron, this fire of which we are talking does with a sordid, icy and hard heart. In fact, as soon as this fire enters, the human soul gradually puts away every darkness, every coolness, every hardness, and it becomes similar in every way to him by whom it is inflamed. By the effect of the flame of divine fire, [the human soul] burns up everything, blazes and is melted in God’s love, according to the apostle’s words: God’s charity has been infused in our hearts by the Holy Spirit’s work who has been given to us.
De Trinitate, Book 6. XIV