Canon John Fenton, the New Testament scholar, once said of Christianity: “You’ve got to feel it in your guts.” Charles I came to feel Christianity - and, in particular, the Church of England - in his guts.
This meant that he was always going to clash with those of his subjects who felt other sorts of Christianity in their guts: Puritans and Presbyterians. After his imprisonment by Parliament, the king had several opportunities to save his life and regain his throne, but the price was always that he should consent to the abolition of the Church of England and its replacement by Presbyterianism.
The king refused. When all else had fallen away, he still believed that the Church of England was the purest form of Christianity, freed from later accretions, and in tune with the ancient Fathers. For this, he was prepared to die.
Source: Did King Charles I have Asperger’s syndrome?