Sunday is Trinity Sunday. Last week was Pentecost. People including preachers sometimes get down in the weeds with trying describe the relationship between the persons of the Trinity or worse trying to describe the nature of the Trinity. The work of the Spirit is wonderfully described by Martin Luther
It is not enough simply that Christ be preached; the Word must be believed. Therefore, God sends the Holy Spirit to impress the preaching upon the heart–to make it inhere and live therein. Unquestionably, Christ accomplished all–took away our sins and overcame every obstacle, enabling us to become, through him, lords over all things. But the treasure lies in a heap; it is not everywhere distributed and applied. Before we can enjoy it, the Holy Spirit comes and communicates it to the heart, enabling us to believe and say, “I too, am one who shall have the blessing.” To everyone who hears is grace offered through the Gospel; to grace is he called, as Christ says (Mt 11, 28), “Come unto me, all ye that labor and are heavy laden,” etc.
The believing heart never sees God with jealous eye. It does not fear being cast into hell as it did before the Holy Spirit came, when it was conscious of no love, no goodness, no faithfulness, on God’s part, but only wrath and displeasure. But once let the Holy Spirit impress the heart with the fact of God’s good will and graciousness towards it, and the resulting joy and confidence will impel it to do and suffer for God’s sake whatever necessity demands.
Let us, then, learn to recognize the Holy Spirit–to know that his mission is to present to us the priceless Christ and all his blessings; to reveal them to us through the Gospel and apply them to the heart, making them ours. When our hearts are sensible of this work of the Spirit, naturally we are compelled to say: “If our works avail naught, and the Holy Spirit alone must accomplish our salvation, then why burden ourselves with works and laws?”
In the Tolkien Classic “The Hobbit”, a great heap of treasure is guarded by Smaug, a dragon. He can’t use the treasure but he doesn’t want anyone else to have it. The devil is described as a dragon. Guarding the treasure of the Gospel and trying to make sure that anyone else doesn’t get any, Jesus has already defeated him, and the Holy Spirit dispenses the treasure of God’s grace.