Part I - Introduction, Definition and Historical Background
The doctrine of the trinity is one of mainstream Christianity's most universally accepted and hallowed doctrines held sacrosanct by Catholics, Protestants, and Orthodox believers alike. This doctrine, as defined by the Second Ecumenical Council, is so set in stone that it has become a commonly used litmus test for defining true belief. However, what is interesting is that not only is the word "trinity" absent from the Bible but other commonly used phrases, such as "three persons", "one essence", "one substance", "three in one," are also curiously missing. The doctrine evolved in the late first century, then gradually gained momentum in the subsequent three hundred years or so, taking final shape in 381 AD. Also the very same scholars who claim that one cannot be saved without believing in the Trinity (although the Bible says nothing of the sort), make no bones about the fact that the doctrine itself is incomprehensible.
Part II - Plurality in The Godhead
The Hebrew Scriptures very definitely point to more than one Person in the Godhead. It is possible that many of those who emphatically declare that a person cannot be saved without a belief in the Trinity may actually have the Deity of Christ in mind. The New Testament abounds with proof of the deity of Jesus Christ, and Acts 5:3-4 is more than enough to clinch the issue of the Holy Spirit being God. However, theologians over simplify the matter when they state that the Biblical evidence permits only one of two conclusion a) Jesus and the Holy Spirit are not Divine, or b) God is a Tri-unity. It's either this or that... take it or leave it.
Part III - The Deity of Christ & The Holy Spirit
Challenging the doctrine of the Trinity does not mean challenging the Divinity of Christ
and it does not mean challenging the Divinity of the Holy Spirit. The New Testament abounds with much proof of the deity of Jesus Christ, and Acts 5:3-4 is more than enough to clinch the issue of the Holy Spirit being God. However, there is absolutely no evidence that the Holy Spirit is the 'third person of the trinity.
Part IV - Passages That Supposedly "Prove" the Trinity
Although there isn't a single verse or passage that "clearly" states that there is one God who exists in three persons, Trinitarians usually advance a number of verses (in less polite terms, the usual suspects are rounded up) as evidence for there being three co-equal members of the Godhead. In reality these verses do no such thing. Read without prejudice or preconceived ideas, the "proof texts" merely state that Father, Son and Holy Spirit exist. None of them can be used to prove the three-in-one theory. Yet, asking a person to read the actual text, and not allow anyone to tell them what they think the text means, seems to be quite a novel concept in Christianity. Why? Are we just so used to having the Bible interpreted for us that we are too lazy or too gullible to go look for ourselves? Or have we been so indoctrinated that when we read the verses in question, we read into them what we have been led to believe they say / have always been told what the meaning is.
Part V - The Grammar
Because the Holy Spirit is referred to as 'He' or "Him" in quite a few places in The New Testament, many people assume that the Holy Spirit is male. In fact, the grammar is often used by many evangelicals as the first line of defense against any challenges to the doctrine. The problem is that the grammar cannot necessarily be used to support the idea that the Holy Spirit is a “he”, much less the third Person of the Trinity. Doctrinal bias, not grammatical accuracy, is responsible for referring to the Holy Spirit with masculine rather than neuter pronouns when both are equally legitimate. In other words, there is no grammatical foundation for the orthodox view of the Trinity.
Part VI - The Holy Spirit... A Separate Person, Or The Divine Presence And Power Of The Father Himself
Not only does the Bible makes it very clear that there is only one Spirit, but there is a huge mountain of evidence that suggests that the Spirit of God the Father, the Spirit of Christ and the Holy Spirit are one and the same Spirit. This becomes even more apparent if you consider how the New Testament authors consistently ignored the "third person of the Godhead". The third person of a supposedly triune God is missing from the opening salutation of most of the New Testament books, from the approximately eighteen doxologies found in these books, and curiously absent from Daniel's, Stephen's and John's visions of heaven.
Part VII - The Cappadocian Fathers
If not the Scriptures, where did the idea of the trinity come from? Three theologians from Cappadocia ... Basil - bishop of Caesarea, his brother Gregory - bishop of Nyssa, and Basil's close friend Gregory of Nazianzus jointly known as the Cappadocian fathers, gave definitive shape to the doctrine of the Trinity. The problem is that not only were all three trained Greek philosophers but they were greatly influenced by the writings of Origen known for introducing Greek ideas into Christianity. Gregory of Nyssa applying Origen's line of reasoning to the trinity claiming we would have no content for our thoughts about Father, Son, and Spirit, if we did not find an outline of their nature within ourselves. In other words, the key to the Trinity is in our triple nature ... our minds or reasoning, our word, and our souls. To top it off, all three were Catholic mystics. Basil and Gregory of Nazianzus pioneered the rules of monastic life, compiling what became known as "the Rule of St. Basil".
Part VIII - The Cappadocian fathers - Begotten vs. Proceeds
The Cappadocian fathers believed and taught that both Son and spirit derive from the father, but are derived in different ways i.e the Son is "begotten" of the Father, and the Spirit "proceeds" from the Father. They apparently missed the fact that Jesus also spoke of 'proceeding' or 'coming from' the Father and that "begotten" was used in an entirely different situation
Part IX - Summary and Conclusion:
The church, substituting their own rules for what the Scriptures teach, has often insisted you believe in their version of the trinity in order to consider yourself as being saved. The truth is that how many Beings the Godhead consists of has absolutely no bearing on salvation. We are no closer to really understanding everything about the Father and the spiritual realm than we are to taking a stroll around Pluto. Make no mistake, the Lord is not going to judge you on whether you are right or wrong about this. He is going to judge you on how righteously you live and whether or not you have followed His commandments.
If you wish to believe there is a trinity, go right ahead. However, if you actually look into the historical and Biblical evidence for yourself, you are very likely to realize to what extent the church has based this doctrine on ignorance of the fact that it stems from pagan sources not the Bible. They then skewed the English translations to reflect this belief then impose the concept of the trinity onto and Biblical passage that mentions Father, Son, and Holy Spirit in one breath. Sadly, this is far from the only erroneous doctrine that is taught as ‘Biblical’. Several more are listed HERE