I saw the following meme on Facebook and I responded to it with what I wrote below it. It's my very brief refutation:
MY RESPONSE:
Nearly Every other doctrine in the Bible needs to be systematized. Whether the attributes of God, eschatology, ecclesiology, sacramentology, atonement theories, how and in what sense the OT laws continue and/or are abrogated under the New Covenant, the relationship [if any] between Israel and the Church, etc. Why think that wouldn't be the case for Christology and theology proper [i.e. the doctrine of God in particular]? Most of the New Testament books are occasional letters. Meaning letters written on an occasion to address a particular issue the author deems necessary to focus on. They weren't written to teach the very basics of the faith.
The rudimentary elements and concept of the Trinity are presupposed in the New Testament Scriptures, and strongly hinted at in the Old Testament. The Trinity makes most sense of the comprehensive data and evidence we find in the Bible. It's the best abductive explanation. Abduction is inference to the best explanation. Or reasoning to that theory that has the greatest explanatory power, explanatory scope, is least Ad Hoc, fits best with already established facts, et cetera.
For example, it only makes sense that the Holy Spirit is in some sense divine if Christians are the temple of God due to the indwelling of the Holy Spirit [1 Cor. 6:19; 3:16-17] . It only makes sense that Jesus is fully divine if He's worshipped to the same degree as the Father [John 5:23; Heb. 1:6 citing the LXX of Ps. 97:7 & Deut. 32:43]. These, and many other thing, are things that don't make sense in various types of anti-Trinitarian Unitarianism.
Why for example does the New Testament teach all three persons dwell in believers [Father and Son: John 14:21-23; Son: Rev. 3:20; John 17:23], if it's not the case that all three persons are divine. Why are ALL THREE said to be INVOLVED in creation [Jesus: John 1:1ff.; 1Cor. 8:6; Col. 1:16; Spirit:Job 33:4; Ps. 33:6; 104:30], and redemption [Spirit: Eph. 4:30; Rom. 8:23], sanctification [Jesus: 1 Cor. 1:2, 30; Heb. 10:10; 13:12; Spirit: 1 Pet. 1:2; 2 Thess. 2:13; Rom. 15:16] and the resurrection of Christ [Christ: John 2:18; 10:17-18; Spirit: Rom. 1:4; 8:11]? Why are all three taught to be followed and obeyed [Jesus is obvious; Spirit: Isa. 63:10; Acts 7:51]? Why all three are said to give spiritual gifts [Jesus: Eph. 4:7ff.; Spirit: 1 Cor. 12:11]? Why all three said to be mentioned and invoked in the initiation rite of entrance into the church [Matt. 28:19ff.]? Why all three invoked in a benediction [2 Cor. 13:14]? Why all three are said to be objects whom we ought not to blaspheme [Mark 3:28-29; Matt. 12:31-32; Luke 12:10]? Why all three objects whom we are to seek fellowship [Jesus: 1 John 1:3; Spirit: 2 Cor. 13:14]? Why all three teach and guide us [Jesus: John 13:14; 8:12; Spirit: John 14:26; Luke 12:12; 1 Cor. 2:12-13, Heb. 9:8; Job 32:8] Why all three are a source of grace [Jesus: 1 Thess. 5:28; 1 Cor. 13:14; Phil 4:23; Spirit: Heb. 10:29; Zech 12:10]? EXAMPLES COULD BE MULTIPIED. But they will never be enough for a hard headed Unitarian.