Acts 2 records the event of God's pouring out of His Spirit on the church, as well as the accompanying manifestations that testified dramatically that something extraordinary was taking place. Subsequently, the Holy Spirit is a significant theme throughout the rest of Acts, as the gospel was preached and more people were called into the church. The epistles of Paul, Peter, and John likewise feature the Holy Spirit frequently. Yet, for all that is written about it, the Holy Spirit is still commonly misunderstood. Many theologians claim to know what the Holy Spirit is, yet they simultaneously profess it to be an incomprehensible mystery!
Part of the difficulty in understanding God's Spirit comes from the common challenges that arise whenever a text is translated from one language, with all of its nuances, into another. In this case, the Greek word translated as “spirit” is pneuma. E.W. Bullinger, in The Companion Bible, catalogs fourteen different meanings or usages of that one Greek word. It should not be surprising, then, that when Greek texts are concisely translated into English, some of what is intended by pneuma can become clouded.
Further confusion has been introduced by the so-called “early church fathers,” whose writings are often looked to for guidance in understanding early Christian doctrine. They may have been early on the scene, yet they were also influenced by Greek philosophy, Plato in particular. Plato's worldview—one not based on the Bible—promoted a triune godhead or a single god that mysteriously expresses itself in three different persons or personalities. Plato himself developed this view from much older trinities found in the Babylonian mystery religions, as well as Egyptian beliefs.
One of the rarer usages of the word pneuma is “a spirit being,” thus it was not a great leap for early scholars—looking through a lens of pagan concepts—to regard the Holy Spirit as a third God-Being. Because those involved were already inclined to think in terms of a god consisting of three persons, they were able to find “evidence” of such an idea in the Scriptures.
It has been said that heresy crawls in its first generation, it walks in the second, and then it runs. Once the notion of the Holy Spirit being a third person got its start, it walked and then soon sprinted throughout the Western world with such force that now the overwhelming majority of professing Christians take the idea as a given.
It is worth remembering that there is indeed a spirit being striving for equality with the Father and the Son, but that spirit—Satan the Devil—is anything but holy (Isaiah 14:13-14). He has, though, created a place for himself in the minds of millions by guiding Catholic and Protestant doctrine to include a mysterious third spirit being within a three-part godhead, just as the ancient pagan religions held. Yet, that construct is nowhere found in the Hebrew Scriptures, nor is it unambiguously seen in the Greek Scriptures. It is a doctrine that must be read into the Greek text, but doing so only creates contradiction and confusion—neither of which are from God (John 10:35; I Corinthians 14:33).