Now, I've already offered a host of biblical reasons for the doctrine (reasons that precede Nicaea). Unfortunately, I've also offered the caution of playing the "For the Bible tells me so" card too easily. And we're all aware that "lots of Scriptures" doesn't equate to "necessarily true." So, your Honor, I'd like to offer a couple of other pieces of evidence.
Did you know that the Emperor Constantine was not a believer in the Trinity? Truth be told, Constantine favored Arianism. There was an argument in the Church, so Constantine I called the very first Church Council together to unify the Church. The primary question was the nature of Jesus. The outcome was the Nicene Creed, which includes this statement about Christ: "very God of very God, begotten, not made, being of one substance with the Father". Arius denied this and claimed that Jesus was made, not of one substance with the Father. The Council voted some 316 to 2 against Arius. Constantine's role in all of this was to call the Council and to pay for some of their travel and lodging expenses. As it turns out, Constantine himself supported Arianism, but felt that unity in the empire was more important than his view and exiled Arius. Several times in later years he attempted to bring back Arius and his beliefs, but the subsequent Councils continued to deny the Arian doctrine as valid and the Church officially adopted the doctrine of the Trinity against Constantine's personal preferences.
The doctrine of the Trinity, however, predates the Council of Nicaea. This is a key point that so many detractors seem to fail to comprehend. The Council of Nicaea didn't bring it about; it was already in place. The Council of Nicaea didn't make the doctrine; it defended it against a challenge.
Take a cruise around the Internet sometime. You will find several sites that offer a variety of quotes from sources as early as the first century that assume or defend the doctrine of the Trinity. From Polycarp to Irenaeus to Tertullian to Origen, all of whom precede the 4th century, there is a host of references to the Trinity. Indeed, it was the lawyer Tertullian who first coined the term "persons" in reference to the three "persons" of the Trinity. The doctrine historically existed and was assumed as true centuries before it was affirmed at Nicaea despite the best arguments of its detractors.
In closing, your Honor, I'd like to offer one last piece of interesting evidence. It's somewhat outside of the normal realm of the argument, but I think it is pertinent. There are those who argue that the Aramaic Peshitta version of the Bible is the original form. They argue that, rather than being written first in Greek, it was written first in the language of the day, Aramaic, then translated to Greek. There are several interesting reasons for the argument and I don't intend to offer them here or settle that dispute. However, one of the differences between Aramaic and Greek is found in the solely Hebraic word for the Name of God: YHWH. In the Old Testament translations we often find the word YHWH translated as "LORD", where the letters are all capitalized to signify that it is the Tetragramatton -- YHWH. In Greek, however, there is no such possibility. So what if an author of a New Testament book intended that usage? Well, they would have used YHWH in their original Aramaic version, but it would have been translated to kurios in the Greek version. Interestingly, in the English translation of the Aramaic Peshitta version of the New Testament we read this:
No one can say, "Yeshua is YHWH," except by the Set-Apart Spirit (1 Cor. 12:3).It would appear that one of the earliest known versions of the New Testament assumes an equivalence between "Jesus" (Yeshua) and Jehovah ("YHWH"). That is Trinitarian at its heart.
The next time you hear that Constantine interjected the Trinity into the Church, shake your head and walk away. It has been repeated enough that you will be thought a fool do deny it ... but it isn't true. The historical facts don't support it. Your Honor, I object. Assumes facts not in evidence. Instead, all the evidence -- both biblical and historical -- supports the doctrine of the Trinity from the start of Christianity.