[The following article by Tim Barnett is from the revised CSB Apologetics Study Bible for Students. Sean McDowell describes all the additions and updates in the new edition here.]
Few doctrines of the Christian faith are more essential than the Trinity, yet many people are deeply confused about its nature. Sadly, this can prevent individuals from truly knowing God (Jn 17:3).
What the Trinity Is
The Trinity is the belief that there is one God, who subsists as three distinct, co-equal, co-eternal persons. While God is one Being, He is also three separate, divine persons.
Though you won’t find the word Trinity in the Bible, the concept is biblical. This doctrine emerges when all of Scripture is surveyed and three essential elements come together. First, there’s only one God. Second, the Father, Son, and Holy Spirit are distinct persons. Third, each of these persons is fully God.
Element #1: From Genesis to Revelation, the Bible clearly affirms monotheism—that there is only one God. Every morning the faithful Jew would repeat a prayer known as the Shema: “Listen, Israel: The Lord our God, the Lord is one” (Dt 6:4). Isaiah also speaks with clarity that there is no God but one (Is 43:10; 44:6; 45:5; see 1Co 8:4). Jesus too affirms this belief when explaining the greatest commandment (Mk 12:29).
Element #2: The Father, Son, and Holy Spirit are clearly distinguished from each other by the way they interact with one other in personal ways. For example, at Jesus’s baptism, as the Holy Spirit descends on the Son, the Father says, “You are my beloved Son; with you I am well-pleased” (Lk 3:22).
Element #3: All three persons of the Trinity are fully God. The Father is repeatedly called God (1Co 8:6; 1Pt 1:3). Paul writes, “Blessed is the God and Father of our Lord Jesus Christ” (Eph 1:3). The Son is called God on numerous occasions (Jn 1:1; Rm 9:5; Ti 2:13–15; Heb 1:8; 2Pt 1:1). For instance, Thomas boldly calls Jesus, “My Lord and my God” (Jn 20:28). Finally, in the inception of the church, Ananias and Sapphira dropped dead after lying to the Holy Spirit since they had “not lied to people but to God” (Acts 5:1–4).
Therefore, if there is only one God, and if the Father, Son, and Holy Spirit are distinct persons, and if all are affirmed to be fully God, then the only explanation that makes sense of all the biblical data is the Trinity.
What the Trinity Is Not
The Bible does not teach that three beings are one being, or that three persons are one person. The Bible teaches that one divine Being is also three persons. Certainly this is a mystery, but it’s not a genuine contradiction. While beyond reason, it’s not against reason (Dt 29:29).
The Trinity and God’s Love
Most people agree with 1 John 4:8: “God is love.” Love is one of the moral perfections that are essential to God’s nature. Without love, He wouldn’t be God. Love necessitates a deep affection for another. But how could God truly be love before he created anyone to love? Only the Trinity makes sense of the perfect, eternal love of God. It is this love that invites us into relationship with Him.