This error comes through what is taught in their version of the Bible (The New World Translation). This is their version of the Christian Greek Scriptures which says...
(NWT) John 1:1 "... and the Word was a god."
(KJB) John 1:1 "...and the Word was God."
Isaiah shows this to be false in 43:10 "...before me there was no God formed, neither shall there be after me." (KJB)
Isaiah 44:6 "Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I am the first, and I am the last; and beside me there is no God." (KJB)
Isaiah 45:5 and 21 "I am the LORD, and there is none else, there is no God beside me: I girded thee, though thou hast not known me: ...and there is no God else beside me; a just God and a Saviour; there is none beside me." (KJB) These are other times Jehovah declares the impossibility of another God beside Himself.
**The Jehovah's Witnesses deny the personality and Deity of the Holy Spirit.
Eight times in the Word of God the Lord Jesus refers to the Holy Spirit by the personal masculine "HE". Even the Jehovah's Witnesses own version (NWT) recognizes the Holy Spirit's personality in John 16:13-14 "However, when that one arrives, the spirit of the truth, he will guide YOU into all the truth, for he will not speak of his own impulse, but what things he hears he will speak, and he will declare to YOU the things coming. That one will glorify me, because he will receive from what is mine and will declare it to YOU." (NWT)
(KJB) I Corinthians 12:4-6 "Now there are diversities of gifts, but the same Spirit. And there are differences of administrations, but the same Lord. And there are diversities of operations, but it is the same God which worketh all in all."
(NWT) I Corinthians 12:4-6 "Now there are varieties of gifts, but there is the same spirit; and there are varieties of ministries, and yet there is the same Lord; and there are varieties of operations, and yet it is the same God who performs all the operations in all persons.
Notice in their translation that the Holy Spirit is called Lord and God. Isn't that interesting? They say there is only ONE God....God Jehovah. But they fail to realize that even their own teachings tell that there is ONE God in three Persons. Here we see their translation telling them that the Holy Spirit is God.
If you compare Isaiah 6:8-10 with Acts 28:25-27 in their own translation you will see that their own teachings say that the God of Isaiah is indeed the Holy Spirit of the book of Acts.
(NWT) Isaiah 6:8-10 "And I began to hear the voice of Jehovah saying: 'Whom shall I send, and who will go for us?' And I proceeded to say: 'Here I am! Send me.' And he went on to say: 'Go, and you must say to this people, 'Hear again and again, O men, but do not understand; and see again and again, but do not get any knowledge.' Make the heart of this people unreceptive, and make their very ears unresponsive, and past their very eyes together, that they may not see with their eyes and with their ears they may not hear, and that their own heart not understand and that they may not actually turn back and get healing for themselves."
(NWT) Acts 28:25-27 "So, because they were at disagreement with one another, they began to depart, while Paul made this one comment: 'The holy spirit aptly spoke through Isaiah the prophet to YOUR forefathers, saying, 'Go to this people and say: 'By hearing, YOU will hear but by no means understand; and, looking, YOU will look but by no means see. For the heart of their people has grown unreceptive, and with their ears they have heard without response, and they have shut their eyes; that they should never see with their eyes and hear with their ears and understand with their heart and turn back, and I should heal them.'"
We know from the Word of God (KJB) that the Holy Spirit is indeed God but even the Jehovah's Witnesses own translation teach that very same thing. The God of Isaiah that they so strongly hold to is the same God in the Person of the Holy Spirit that is spoken of in the book of Acts in their own translation.
These people are so deceived and lied to in so many areas. I remember a time when talking to a Jehovah's Witness at my door them telling me that they used the KJB to study from and get their learning. They have since come away from that as far as I know....since now they use the New World Translation.
Here is something I found in my research:
Jehovah's Witnesses have their own Bible since they cannot accept the theology and teachings of the original scriptures which are handed down to us. The many items of Christian teaching which they reject are reflected in the alterations they make to the text.
The fact is that The New World Translation is not accepted as an accurate and faithful translation of the original Hebrew and Greek scriptures by almost all theologians since its conception, whether Christian or not. Only a very small handful accept its alternative position. Otherwise, the only supporters of the translation are Jehovah's Witnesses themselves. They reject concepts such as the Trinity as the work of the devil, and reject the divinity of Christ.
While the actual word 'Trinity' does not appear in any translation of the Bible, the actual concept of the Trinity - God as Father, Son and Holy Spirit, and also the divinity of Christ, are plainly seen in every other translation, in the original texts, and in writings right back through the early Church, through even to Polycarp who was a disciple of John the apostle himself - Jesus' closest friend.
However, the New World Translation was translated so as to remove any reference to the divinity of Christ, and the Trinity, (as just one example, see John 1 'In the beginning was the Word (Jesus), and the Word was with God, and the Word was God..." (King James Bible) - compared with "In the beginning was the word, and the word was with God and the word was a god" (New World translation) despite the original Greek using the word theos - meaning God rather than 'a god'.