When did Saint Patrick become a Christian?

Answer 1

Anyone who is in heaven is a saint, including the angels. Canonization is just the official process by the Church where this is publicly recognized and a Mass and Office are established for a particular person. However, this is rather recent:

St. Patrick died, and entered heaven, and has been celebrated as a saint long before the Church's process of canonization was even conceived.

Answer 2

There was no canonization process in place when Patrick died, that came about in the 12th century. He would have been declared a saint by popular acclamation and his sainthood approved by a local bishop soon after he died in 467. Today only the pope can declare a saint and only after a lengthy canonization process.

Answer 3

He started his 15 years of religious training in around 409/410 when he returned to Britain after an angel spoke to him in his dream telling him to return to Ireland as a missionary.

Answer 4

Patrick was a Catholic from birth as his family was Catholic. The father of Patrick, Calpurnius, was a deacon in the Church and Patrick's grandfather is said to have been a priest.