As well as anyone can. The Trinity is the fundamental mystery of Christianity. As such, a definitive understanding is beyond our comprehension! That doesn’t mean it’s useless to try to conceive of the nature of God. It just means we approach the subject with great humility when we do.
Consider Trinity as the specifically Christian way of talking about God. When we meet God face-to-face in eternity, it may not be the best word to describe the encounter, but for now it will have to do. At the center of our faith is this doctrine: We believe we are rescued from sin and death by God through Christ by the power of the Holy Spirit. As theologian Catherine Mowry Lacugna put it, “God, Christ, and the Spirit are equally responsible for our salvation,” and each is divine.
The religion of Israel spoke of God as “One,” and not multiples, quite emphatically. Monotheism was a prize Jewish contribution to Near Eastern religious thought, and Christianity says nothing to disagree or to dislodge its significance. Yet even in Hebrew scripture God is variously known and depicted as Spirit (in the breath of creation), Word (in law and prophecy), Presence (in the Tent of Meeting during the Exodus years), and Wisdom (in the wisdom books). That doesn’t carve up the divine nature so much as give us poor mortals a way of speaking about Infinity without getting a headache.
In the person of Jesus, humanity encounters God in a way as intellectually groundbreaking as when Moses came into relationship with God on Mt. Sinai,or the prophets received oracles and revelations. Both the Incarnation and Pentecost reception of indwelling Spirit changed the way we know God for all time. It’s no wonder doctrines about the Trinity emerged by the 4th century, countering other ideas we now call heresies which attempted to subordinate Jesus to something less than a full participation in divinity.
In 1442 the Council of Florence affirmed God the Father as “Unbegotten” (coming from no source and without beginning), the Son as “Begotten” but not made, and the Holy Spirit as “Proceeding” from Father and Son (being sent by and rooted in both). The interior relationship of the Trinity is such that we can’t really speak of separate realities, anymore than I can talk about my mother, my father’s wife, and the woman she is in herself as three people. Who God is for Christians is Trinity. Who God is to God is still a mystery.
• Deuteronomy 5:6-10; 6:4; Matthew 3:16-17; 28:19; Romans 8:14-16; 2 Corinthians 13:13; Galatians 4:6; Ephesians 1:3-14
• God for Us: The Trinity and Christian Life by Catherine M. Lacugna (HarperOne, 1993)
• The Trinity (Kindle edition) by Anne Hunt (Liturgical Press, 2010)
• “What on Earth is the Trinity? The Trinity in Everyday Life” by Jeremy Ive