So generally speaking, Charles Stuart is an unusual person to be considered a Saint. None of the Stuarts are memorable for being good Kings. Particularly after Queen Elizabeth, King James and King Charles had big boots to fill. And after the Interregnum of Oliver Cromwell, King Charles II dissolved Parliament and ruled as an absolutist, and King James II was driven from England by a Dutch Invasion, and never had his throne restored to him. The Stuarts were not the best of kings.
Looking at Charles specifically, he sought to rule as an absolutist monarch, he was considered to be widely unpopular among his people, he interfered with the Church of Scotland, attempted to negate the powers of the Scottish and English parliaments, continued to oppress Ireland and he presided over the English Civil War in which he ultimately lost and was beheaded on charges of tyranny.
When we look at these things, it seems on paper that to have Charles be considered saintly. Especially when we put him into the company of people like Mother Teresa, St Francis, or anyone we first think of as saints.
There has been a long history of veneration of figures within Christianity of royalty that may or may not have been the best of people. Emperor Constantine XI (Byzantine Empire), Emperor Nicolas II (Russian Empire), King Kamehameha IV (Kingdom of Hawaii), King Dagobert II (Merovingian France), and Emperor Henry II (Holy Roman Empire). Some are declared saints, others (either for political reasons or because of when and where they lived) are only remembered and venerated in prayer. These kings were remembered for their faith, but they were also made mistakes. And in the end they died, some in battle, others executed, or quietly and slowly forgotten.
History will often follow the lives of leaders, royal or otherwise, because they are visible. And because of that, more attention is given to their lives, both good and bad. For kings, there are fewer myths and more criticisms within history, particularly as history has developed to be a more critical study. But things do not exist in a vacuum, a person’s legacy far outlives them, and even more so for a king. We all make mistakes, and the magnitude of the mistakes a king can make is far greater than that of a normal person. But the magnitude of good can also be greater than a normal person as well.
So why should we remember and venerate Charles Stuart as a Saint? That is a hard question. To be sure, his legacy endures because the Church of England never adopted the Puritanical reforms of Oliver Cromwell, namely a Presbyterian polity. A Catholic character remained in Anglicanism, and it was grown by the Oxford Movement to really encapsulate the whole of Anglicanism. But all of these things, Charles did not do. They all happened after his execution. His attempts in his life to do what he wanted ultimately drove England apart.
His legacy begins with the choices he made. He was offered the chance to retain his throne and life if he abolished the episcopate, and he refused. For that, he died. It was seemingly for naught since Cromwell went and abolished it any way. And though the episcopate suffered during the Commonwealth, it survived and was able to establish itself once more during the restoration. A small minority in England and Ireland resisted Cromwell, and suffered for it. And though Charles did a lot that was wrong, it was in the moment of truth that he showed his convictions and faith.
This is where martyrdom, sainthood, and faith all converge, in the moment. Many saints lived lives that were questionable. St. Paul, St. Genesius, and others lived terrible lives, made mistakes, hurt others, and though may have done good as well, they chose at the moment of truth to stand by faith and God. Charles was not perfect, he was not a very good king, Monty Python laughs about how "The most interesting thing about King Charles the First is that he was five foot six inches tall at the start of his reign, but only four foot eight inches tall at the end of it," and that "in spite of his intelligence and cultivation, Charles was curiously inept in his contacts with human beings. Socially, he was tactless and diffident, and his manner was not helped by his stutter and thick Scottish accent, while in public he was seldom able to make a happy impression." Despite that, Archbishop William Laud describes him as "A mild and gracious prince who knew not how to be, or how to be made, great." And maybe, just maybe, that humility was what helped him say yes when the question came to him.
Thus, St. Charles Stuart, King and Martyr, lives on and reminds us that even though we are imperfect, we are still capable of saying yes to God.