by John MacArthur
Why does the doctrine of the Trinity matter to us today? And why have so many great Christians throughout church history fought so tenaciously in defending it? The answer is fundamentally rooted in one critical question: Do we know God?
Jesus said that knowing God is synonymous with having eternal life (John 17:3). And if we define Him on any terms other than how He has defined Himself in Scripture, we are nothing more than idolaters. That is why sects like the Jehovah’s Witnesses and Mormons are regarded as cults. To inherit eternal life, we need to know God as He truly is. And the biblical testimony is clear: There is one God. He eternally exists in three persons. And all three persons are each fully God.
To put it another way, God is three distinct persons in one indivisible substance. In the words of the Athanasian Creed,
The Father is God, the Son is God, and the Holy Spirit is God. And yet they are not three Gods but one God. 
And in this Trinity none is before or after another; but the whole three persons are co-eternal together and co-equal. So that in all things, the Unity in Trinity and the Trinity in Unity is to be worshipped. 
The simplest way to comprehend the Trinity is to read the Bible from the beginning to the end. The word for God in Genesis 1 is “Elohim.” It is plural. The im ending on a noun in Hebrew is like s in English. The opening words of Genesis could be translated, “In the beginning, Gods.” The word-form of the noun is plural, and yet the reference is to a singular being. The description of God throughout the Old Testament is clearly to a singular being. The verb that goes with Elohim in Genesis 1:1 is likewise singular.
The benediction God gave Moses for the priests to use seems to allude to the Trinity. Three times they were to invoke the blessing of the Lord. Numbers 6:24–26 records it: “The Lord bless you, and keep you; the Lord make His face shine on you, and be gracious to you; the Lord lift up His countenance on you, and give you peace.” The threefold appeal to “the Lord” suggests the Trinity. The seraphim Isaiah saw and described in Isaiah 6 cried to one another with this threefold exclamation: “Holy, Holy, Holy” (Isaiah 6:3). Again, it seems to be an allusion to the Trinitarian nature of God.
One of the clearest Old Testament references to the Trinity is Isaiah 48:16, a prophetic verse spoken by Jesus Christ. It puts all three members of the Godhead together in one verse: “And now the Lord God has sent Me, and His Spirit.”
Repeatedly the New Testament refers to the Father, Son, and Holy Spirit together in the same passage, on the same level. In Matthew 3 we are told that as Jesus was being baptized, the Holy Spirit descended as a dove, and the Father said, “This is My beloved Son, in whom I am well–pleased” (Matthew 3:17). In John 14:16–17, Jesus says, “I will ask the Father, and He will give you another Helper . . . that is the Spirit of truth.” Jesus told His disciples to baptize “in the name of the Father and the Son and the Holy Spirit” (Matthew 28:19). In 1 Corinthians 12 the apostle Paul says, “Now there are varieties of gifts, but the same Spirit. And there are varieties of ministries, and the same Lord. There are varieties of effects, but the same God” (1 Corinthians 12:4–6). The final verse of 2 Corinthians says, “The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit, be with you all” (2 Corinthians 13:14). First Peter 1:2 says that believers are chosen “according to the foreknowledge of God the Father, by the sanctifying work of the Spirit, to obey Jesus Christ and be sprinkled with His blood.”
God is one, yet He is three.