At the time of writing the Nicene creed in 325 CE, why did different Christians have varying beliefs about Jesus Christ and the Trinity?

Christians had gotten along for about three centuries. with a general understanding that

the Father is God;

the Son (Jesus) is God Incarnate; and

the Holy Spirit is God.

But they hadn't actually tried to sort that out too specifically. They had started to refer to the three as a triad.

A lot of Christians' beliefs were closely connected to traditional understanding passed down from the Apostles, who of course were all Jews. The degree to which Jewish ideas were attached to what was passed down apparently varied.

In the early fourth century, a priest named Arius started writing and publishing devotional poetry, and it became quite popular. That drew attention to the fact that, as his poetry made clear, he regarded Jesus as a separate and subordinate god. His was a polytheistic version of Christianity. This was okay with a lot of Christian converts, who had backgrounds in Greek and Roman thought, but those who were conscious of the Jewish tradition of monotheism objected.

The need to come up with a single declared official belief about this was partly because a new Emperor, Constantine, had made Christianity a favored religion, ending his predecessors' persecutions. But he was, basically, a Roman general, and therefore valued organization and clarity. So he leaned on Christian leaders to get their theology sorted out, and he called an ecumenical council (the first since the first-century meeting in Jerusalem, described in Acts 15) to do that.

He was probably surprised they chose the monotheistic option. Most of his direct Christian connections were on the Arian side, including his son's tutors. And it took a few decades to sort out HOW the notion of one divine Being and three divine Persons would work--the concept we now call the Trinity. There was at least one revision of the Nicene Creed along the way ("Nicene" having become the term for people who supported that first council's decision).

The grand Arian controversy had opened.

That this should have been possible at all, three centuries after Christ, shows how slow the Church had been towards exact dogmatic definition; it had been, and always has been, engaged on something else.

-- Charles Williams, "Descent of the Dove: A Short History of the Holy Spirit in the Church"

You had various early church leaders with different teachings. Most taught the Trinity as truth, but others taught differently. Each teacher was basically a law unto himself. Prime among these was Arius, who vehemently denied the Trinity. The Council of Nicea in 325 was called to come to an agreement on universal Christian truths. Out of this, Arius and his supporters came to be branded as heretics for their denial of Jesus Christ and the Holy Spirit as part of the Godhood.

By reciting the rosary each day with care, a person can know many truths about Jesus and the Trinity.

It was then as it is today, many call themselves Christians who are not.

Why is that of any consequence to you when you have the truth available?

For 3 centuries Christians have been scattered by the Mediterranean,

No reference documentation besides the Torah,

Gospels loosely copied one onto the other, every preacher/Apostle carrying his own version,

Unifying the dogma/teaching was not the central authority priority (St Peter and successors)...

Meanwhile, theologians, scholars, and philosophers were trying to deal with complex concepts like how a unique god can have 2 or 3 appearances? hence the concurrent concepts of Trinitarianism, Binitarianism, Monarchianism, Unitarianism etc. even Arianism: an intentionally simplified non-trinitarian version directed to the uneducated Germanic tribes, and a probable draft for later Islam.

In almost 300 years, each people around the Med felt entitled to the beliefs they have been taught for centuries by then.

As an Empire, Rome decided to unify all the peoples under a unified religion. "Experts" had to (arbitrarily?) sort 4 Gospels among dozens (I think the earliest 1st hand written was the main criteria), and to call "heretical" the teachings they didn't select.

We Christians are not interested in the Nicene creed, because all true Christians believe in the word of God, who attests to his Deity. All those who do not believe are just religious, their religion is based on the Antichrist!

Do you know where the antichrist end up? To hell!

For the same reason that there are 30,000 different versions of christianity today.

There are no objective means of verifying most religious ideas, so there are always going to be differing beliefs about them.

Both the Creed and the Bible were created to stamp out heresies which had arisen from arrogance and/or ignorance.

Newsflash: People STILL have have varying beliefs about Jesus Christ and the Trinity.