One more bit from Letham:
Indeed, the Christian faith can be summed up as, inter alia, a series of unions. There is the union of the three persons in the Trinity, the union of the Son of God with our human nature, the union of Christ with his church, the union established by the Holy Spirit with us as he indwells us. Each of these unions preserves the integrity of the constituent elements or members, being at once a real union and simultaneously not absorbing the one into the other.
—Union with Christ: In Scripture, History, and Theology, pp. 37-38
You could spend the rest of your Christian life simply studying the various unions mentioned by Letham and still not come to the end of the grace and glory of the Gospel. The Triunity of God alone ought to, and will, delight us for all eternity, before we even get to the unions whereby God includes us in that life, becomes our brother, makes us part of the church, or sets us free from sin. Simply put: the Gospel is a union thing.
Soli Deo Gloria