Mormons and Jehovah Witnesses, must believe in more than one "God" or Jesus is not "a god" as they teach. They reject the Triunity of God as unreasonable. I believe that John got it right by not including the definite article.
In Greek to exclude the definite article (the) implies the indefinite article (a, an). But to include "the" would mean that John was saying that Jesus was the only God. To exclude it does not automatically mean to include the indefinite article "a" god. That would mean that John was saying there are more than one God. Again, a misinterpretation as the Bible clearly states there is only One God:
"Hear, O Israel: The LORD our God, the LORD is one!"
Isaiah 43:10, 11
"You are My witnesses," says the LORD, "And My servant whom I have chosen, That you may know and believe Me, And understand that I am He. Before Me there was no God formed, Nor shall there be after Me. I, even I, am the LORD, And besides Me there is no savior." (See Titus 1:3,4)
"Do not fear, nor be afraid; Have I not told you from that time, and declared it? You are My witnesses. Is there a God besides Me? Indeed there is no other Rock; I know not one." (See Isa. 44:6, 45:5,6,11,21)
"Yet I am the LORD your God Ever since the land of Egypt, And you shall know no God but Me; For there is no Savior besides Me".
"So the scribe said to Him, 'Well said, Teacher. You have spoken the truth, for there is one God, and there is no other but He.'"
You believe that there is one God. You do well. Even the demons believe; and tremble!
This makes you wonder why the devil teaches man that there many gods. Satan comes as an angel of light. Sometimes he perverts the nature of God, sometimes he just perverts the Gospel. Sometimes he perverts both. Lying devils even believe some truth.
1 Corinthians 8:4
"... we know that an idol is nothing in the world, and that there is no other God but one."
Although God is only one, He is in three Persons, the same one God:
In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through Him, and without Him nothing was made that was made. (Compare Isa.45:18,21)
Although this verse is verbally parallel with Genesis 1:1, the contexts assign temporal differences. Whereas Genesis 1:1 speaks of the beginning of creation and John 1:1 emphasizes the incarnate manifestation of God in Christ, John in this verse establishes the preexistence of Christ in eternity past. He already "was" when the beginning took place. The Prologue to John's Gospel (vv. 1-18) represents one of the most crucial passages in the Bible and is, without doubt, the definitive exposition of the Person of Christ. Jesus is called the "Word". In Genesis, God SPOKE creation into existence.
"Then God said, 'Let Us make man in Our image, according to Our likeness; let them...'"
The phrase "Let Us" describes God contemplating the special creation of mankind. The plurality as well as the unity of God is implied by the text. "Man" (^adam, Heb.) is shown by the plural, "let them."
"Image" and "likeness" are synonyms, the former perhaps stressing representation, and the latter, similarity. This says nothing about man's physical nature, but it points solely to the spiritual and moral nature of mankind, i.e., mind, emotion, and will.
The body is designed for giving expression to this spiritual and moral self. God created mankind so that it would be capable of fellowship with Him.
"Come near to Me, hear this: I have not spoken in secret from the beginning; From the time that it was, I was there. And now the Lord GOD and His Spirit Have sent Me."
"Therefore take heed to yourselves and to all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood."
1 Timothy 3:16
"And without controversy great is the mystery of godliness: God was manifested in the flesh, Justified in the Spirit, Seen by angels, Preached among the Gentiles, Believed on in the world, Received up in glory."
1 John 5:7
"For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one. And there are three that bear witness on earth: the Spirit, the water, and the blood; and these three agree as one."
Some Bibles don't include this passage assuming that it was added after the fact. The NKJV that I use has included it so I quote it as God's Word to demonstrate His triune uniqueness. These three are one in heaven and agree as one on earth.
Acts 5:3, 4
"But Peter said, 'Ananias, why has Satan filled your heart to lie to the Holy Spirit and keep back part of the price of the land for yourself? While it remained, was it not your own? And after it was sold, was it not in your own control? Why have you conceived this thing in your heart? You have not lied to men but to God."
Some religions will say that he lied to the Spirit of God not God the Spirit.
Many mock the idea of the Trinity. They ask how Jesus could have prayed to the Father if Jesus was the God. Those that ask such things do not understand the doctrine of the Trinity. The Biblical doctrine states that God is three Persons yet He is one God.
"And I will pour on the house of David and on the inhabitants of Jerusalem the Spirit of grace and supplication; then they will look on Me whom they pierced. Yes, they will mourn for Him as one mourns for his only son, and grieve for Him as one grieves for a firstborn."
"For unto us a Child is born, Unto us a Son is given; And the government will be upon His shoulder. And His name will be called Wonderful, Counselor, Mighty God, Everlasting Father, Prince of Peace."
"Philip said to Him, 'Lord, show us the Father, and it is sufficient for us.' Jesus said to him, 'Have I been with you so long, and yet you have not known Me, Philip? He who has seen Me has seen the Father; so how can you say, "Show us the Father"? Do you not believe that I am in the Father, and the Father in Me? The words that I speak to you I do not speak on My own authority; but the Father who dwells in Me does the works. Believe Me that I am in the Father and the Father in Me, or else believe Me for the sake of the works themselves.'"
(See also: John 5:18 and Philippians 2:5.6, Rev.1:17 and Isa.44:6)
Is The Trinity Unreasonable?
The Trinity is a Biblical doctrine. God is impossible to understand wholly. It is arrogant to assume that man can understand without apparent contradiction and paradox, the whole nature of God. (Isa.55:8,9, Rom.11:33,34)
This does not mean God does not desire that we reason with Him. He means for us to reason as high as we can. However it is most reasonable to expect limitations upon our reason when dealing with God. Man in his pride does not like limitations. Nevertheless God has even revealed in nature many a "paradoxes", that man may see his smallness:
Do other religions teach a triune God? No! Some teach three or more gods, most represented with a mother, or goddess as they cannot fathom creation other then being birthed.
"For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse,"
This verse teaches that nature reveals the Godhead. Since God is a Trinity higher than reason, we could expect to find the same in nature. Nature is divided into a unity of three's: Time is past, present and future, matter is wave, particle and energy, dimension is height, width, and depth. Modern physics has revealed many paradoxes of nature.
Let us accept the mysteries of the triune Godhead humbly!
In His service