One trivia about the Donation of Constantine:
It is the only document where the words Vicarius Filii Dei can be found in the Papal sense, but even in here, the title is used for Saint Peter.
The Pope is referred to in this document as Vicarius Petri (Vicar of Peter).
Actually, the whole Constantine miraculously cured of his leprosy and baptized by Pope Sylvester, and in gratitude gave him dominion over Rome, Italy and the whole Western Empire story came here.
The Emperor Otto III denounced it as forgery, and Dante Alighieri lamented it as the root of papal worldliness in his Divine Comedy.
It was in the 15th Century, with the revival of Classical scholarship and textual critique that the Church herself now began to doubt the Authenticity of this Document.
Lorenzo Valla then proved in 1440 that the Donation must be a fake analyzing its language, and showing that while certain imperial-era formulas are used in the text, some of the Latin in the document could not have been written in the 4th century.
For example, Constantine’s satraps were referred in the text, though there were no such Roman Officials.
Constantinople was also mentioned, but it was not yet founded at the supposed time of writing and its position as “chief seat” was two centuries away.
Also, the purported date of the document is inconsistent with the content of the document itself as it refers both to the fourth consulate of Constantine (315) as well as the consulate of Gallicanus (317).
About Constantine’s baptism,
It was Eusebius of Caesarea who recorded that Constantine was baptized shortly before his death in May 337, following one custom at the time which postponed baptism till old age or death.
It was St. Jerome who mentioned that Constantine chose to be baptized on his deathbed by Eusebius of Nicomedia, who despite his being a supporter of Arius, happened to still be the Bishop of that region (Constantinople) at that time.
About Eusebius of Nicomedia:
Eusebius of Nicomedia, originally Bishop of Berytus (Beirut), then of Nicomedia where the imperial court resided, then of Constantinople from 338 up to his death, was distinctly related to the Imperial Family of Constantine, whom he owed not only his removal from an insignificant to the most important episcopal see to his influence at court, but the great power he wielded in the Church. He had the complete confidence of Constantine and Constantius II, excepting that time he was exiled in his defense of Arius.
He also was a pupil of Lucian of Antioch like Arius and it is possible that he originally held the same ideas as Arius. He afterward modified his ideas somewhat, or perhaps he only yielded to the pressure of circumstances, but he was, if not the teacher, at all events the leader and organizer, of the Arian party.
He signed the Confession at the Council of Nicea in 325, but only after a long and desperate opposition.
The Emperor was angered by his support of Arius and sent him to exile a few months after the Council, but eventually regained Imperial favor after three years. On his return in 329, he then tried to impose his views upon the Church by bringing the whole machinery of the state government into action.