The Catholic Church naturally admits all historical facts, including the historical fact that they have existed since they were founded in the year 33 A.D.
There are numerous saints, and extant writings from the three centuries that the Church existed until Constantine allowed Christians to legally practice their faith. Why would anyone question those facts?
He started the Roman Catholic Church.
Constantine is not considered a saint in the Catholic Church, only in the Orthodox Church.
You may read a full discussion of Constantine at the link below. To the best of my knowledge the Catholic Church, which is the Mystical Body of Christ made no concessions to Constantine, rather, Constantine's mother was a Catholic, and Constantine was a catechumen, converting on his deathbed. Constantine, also legalized Christianity, and stopped the persecution of the Christians.
Constantine is only considered a saint in the Orthodox Church and was not canonized or declared a saint in the Catholic Church.
Roman is an epithet first commonly used in England after the protestant revolt to describe the Catholic Church. It is rarely used by the Catholic Church. Constantine was Emperor of Rome who removed the penalty for being a Catholic, he, himself, converted before he died, but was not a Catholic for most of his life. He did not divide the Church.
In the Orthodox Church he is. Not in the Catholic Church, though his mother, Helena, is.
Jesus started the first church. All Christians came from the Catholic Church until Constantine says he changed the Sabbath. The came the Dark Ages when the church was divided.
Constantine is not recognized as a saint in the Catholic Church so has no feast day. He is recognized as a saint in the Orthodox Church, however.
Emperor Constantine did not found the Catholic church, Christ himself did through the 1st Pope St. Peter. Constantine converted the pagan Roman empire to Christianity in the 4th century after receiving heavenly signs prior to a battle which he won.
You are confused, first of all, there is no "independent Catholic grouping", there is just the world-wide Catholic Church under the successor of St. Peter. Our Blessed Lord has only established One Church, and, as it is His Body, It will remain ONE until the end of the world. Secondly, Constantine did not select anything, and did not "formalize" the Catholic Church. All Constantine did was to make Christianity legal. The center of the Catholic Church as always been in Rome since St. Peter went there, and died there. Constantine legalized Christianity, but did not become a Christian, himself, until on his death bed.
Jesus started the First Church which was the Catholic Church. Martin Luther started the Lutheran Church.
When King Henry VIII broke away from the Catholic Church and started the Anglican Church in England, and when Martin Luther broke away from the Catholic Church in Germany. This started the Anglican (Church of England, Episcopalian, Anglo-Catholic, etc.) and Lutheran churches.