The way I understand it from my study and seminary education, Constantine was converted because he had a vision that putting the cross of Jesus on his standards (flags) during a battle would lead to victory and it did. When he called the Council of Nicea in 325 C.E., his major concern was a final settlement of the Arian challenge that there was one God the Father and that Jesus was his most perfect creation, but not a "god" of the same caliber as God the Father. Or was Jesus one and the same as God, his beginnings the same and thus equal as the Son of God with God. At the Council of Nicea, this discussion dragged on for a month, but when Constantine arrived, to say that if he was going to have the cross on his standard, Jesus must be a most powerful God. At the beginning of the council, the majority of the bishops were undecided about their positions on this question falling somewhere in the middle. Constantine's regal push coalesced the support into a vote for the 2-gods-in-one approach. When it came to the Nicene Council, The Encyclopaedia Britannica states: “Constantine himself presided, actively guiding the discussions, and personally proposed . . . The crucial formula expressing the relation of Christ to God in the creed issued by the council . . . Overawed by the emperor, the bishops, with two exceptions only, signed the creed, many of them much against their inclination” (1971 edition, Vol. 6, “Constantine,” p. 386). The Holy Spirit seems to have been added in as an after thought. The statement in the creed was simply "We believe in the Holy Spirit" no explanation about how the Holy Spirit meshes with God the Father or God the Son.

At the council of Constantinople in 381 C.E., Emperor Theodosius the Great did approximately the same thing as Constantine had done 56 years earlier. Convene the Council and push his own agenda for his view of the Holy Spirit. “Whether he dealt with the matter clumsily or whether there was simply no chance of consensus, the ‘Macedonians,’ bishops who refused to accept the full divinity of the Holy Spirit, left the council . . . Typically, Gregory berated the bishops for preferring to have a majority rather than simply accepting ‘the Divine Word’ of the Trinity on his authority” ( A.D. 381: Heretics, Pagans and the Dawn of the Monotheistic State, 2008, p. 96). When Greggory, archbishop of Constantinople, who had been charged with "making it happen," fell ill, the a clinker was brought in. “So it was that one Nectarius, an elderly city senator who had been a popular prefect in the city as a result of his patronage of the games, but who was still not a baptized Christian, was selected . . . Nectarius appeared to know no theology, and he had to be initiated into the required faith before being baptized and consecrated” (Freeman, pp. 97-98). The teaching of the Cappadocian theologians “made it possible for the Council of Constantinople (381) to affirm the divinity of the Holy Spirit, which up to that point had nowhere been clearly stated, not even in Scripture” ( The HarperCollins Encyclopedia of Catholicism, “God,” p. 568). And thus was born the triune God...part divine, part political, and...well...made up with very few biblical or logical underpinnings.

By the way, Arian and his followers, because they lost the debate in 325 C.E. were eventually exiled into the "hinterlands" where the idea of 'one god" was kept alive. It wasn't until the 16th Century and the reign of King John Sigismund of Transylvania that Unitarian (one God and Christian) churches began to be founded. Sigismund's court preacher, Frances David had successively converted from Catholicism to Lutheranism to Calvinism and finally to Unitarianism because he could find no biblical basis for the doctrine of the Trinity. And today the debate continues in a much smaller way about which is the "true" Biblical Christianity, Unitarianism or Christianity, both coming from the same source and time.

In the words of Joe Friday, "Just the facts"

[historyforatheists.com]

[en.wikipedia.org]

The "Zoraster religion" gave Christianity many things. Zoroaster, (630-550 BCE - more than 500 years before the man Jesus supposedly was on earth, but a contempary of The Buddha and Confucious) "also known as Zarathustra, Zarathushtra Spitama or Ashu Zarathushtra, was an Iranian prophet whose teachings and innovations on the religious traditions of ancient Iranian-speaking peoples developed into the religion of Zoroastrianism." encyclopedia.com

Zoroaster preached the God of Light and the principle of evil and set the stage for the ideas of montheism and heaven/hell. He taught that truthful humans would have reseraction of the body and a last Judgment. Many paralells have been drawn between Zoroaster and Jesus, but who borrowed from whom is debated yet today.

Absolutely - fertility goddess. Bunnies and eggs anyone?

Did you mean Eostre/Ostara? Could be the same for all I know lol.

Citation?

way back

Yes.

As with many things taken from other religions (cross, pagan holidays Xmas and Easter) it was just easier to adopt something that was already recognized and give a new meaning.