Why is understanding about the Trinity essential for being a Christian?
The word comes from the Latin, "trinitas" which means three or threeness, and the word is not found in Scripture. But is this a problem? No! The Bible from Genesis to Revelation depicts God as being triune in nature, that He has three forms or three ways of revealing Himself in the deepest and most intimate way and means! From the third person case in Gen. 1 & 2, to a more exacting form such as here in Romans 15:17-20! Yet the O.T. proclaims One God only (Duet 6:3-5; Ex. 20; Isa. 40; 44-45), and so does the N.T. (Mark 12:29-30, I Cor. 8:4; Eph. 4:6; I Tim. 2:5)! And grave consequences for not adhering to it! A lot of argumentation over the centuries from the first early church counsel to present day TV preachers misrepresenting God. Why? Because God is beyond the confides of our limited known universe, beyond space and time, and definitely beyond our comprehension! At the same time the Trinity is God's best practice to reveal His Presence to us without a priestly intermediary or some kind of third party.
Thus, God reveals Himself to us as we can be able to understand and comprehend and commune with Him. We tend to throw our own spin and sin into the works and create the arguments ourselves, when God's Word clearly spells out His nature! This doctrine may be hard for most people to understand; nevertheless it is true and essential for us to know the One True God. Basically there is one God who reveals Himself in three Personalities, in theology they are called "Substances."
There is God the Father (Gen 1-2; 6:5-7; Ex. 20; 32:9-14; I Kings 8:27; Psalm 90; 102:25-27; 145:18; Isa. 40:12-31; 66:1; Mal. 3:6; Matt. 10:29-30; John 4:24; 5:26; Acts 4:34-35; 17:23-25; Eph. 1:11; II Pet. 3:8; Rev. 4:10);
God the Son (John 1:-18; 20:28-31; Acts 7:59; Rom. 9:5; 10:9-13; II Cor. 12:7-9; Phil. 2:5-6; Col. 1:15-17: 2:9; Heb. 1:1-12; I Pet. 3:15); and
God the Spirit (John 14:1626; 15:26-27; 16:7-15; Acts 5:3-4).
All three are proclaimed together in Matt. 28:19; Rom. 8; 15:17-29; II Cor. 13:14; Eph. 1:3-14; II Thess. 2:13-14; I Pet. 1:2; Rev. 1:4-5).
These are not separate God's, this would be "Tritheism," nor are they one God acting out three roles, this would be "Modalism". Even though the word "Trinity" as a word is not found in the Bible, however the concept and essence is clearly stated. And the concept is one God with three conversing distinct ways to reveal Himself, "Personalities," or "Substances." They are co-equal and co-eternal, they each have full divinity, yet are a distinct Person. We are required to give equal devotion and worship, but they are not three, but One in purpose and unity condescending to us so that we may be saved (Rom. 5:1-5).
Fons salutis Trinitas! That is a Latin hymn, "Trinity our fountain of Salvation!"