Written in a simple, moving, and straightforward style in the form of a diary, the book combines irenic prayers urging the forgiveness of Charles's executioners with a justification of royalism and the King's political and military program that led to the Civil War.
It is by no means certain that Charles wrote the book. After the Restoration, John Gauden, bishop of Worcester, claimed to have written it. Scholars continue to disagree about the merits of this claim, though assuming that Gauden wrote it, he had access to Charles's papers when he wrote it. Jeremy Taylor is also said to have had a hand in its revision, and to be the source of its title; an earlier draft bore the name Suspiria Regalia, the "Royal Sighs."
Some later editions of the Eikon Basilike contained a sworn statement by William Levett, Esq., longtime courtier and groom of the bedchamber to the King, that Levett had witnessed Charles writing the text during the time that Levett accompanied him in his imprisonment on the Isle of Wight. A witness to the King's execution, Levett later helped transport the King's body back to Windsor Castle for burial.
Whoever wrote the Eikon Basilike, its author was an effective prose stylist, one who had partaken deeply of the solemn yet simple eloquence of Anglican piety as expressed in Cranmer's Book of Common Prayer. The end result is an image of a steadfast monarch who, while admitting his weaknesses, declares the truth of his religious principles and the purity of his political motives, while trusting in God despite adversity. Charles's chief weakness, it says, was in yielding to Parliament's demands for the head of the Earl of Strafford; for this sin, Charles paid with his throne and his life. Its portrait of Charles as a martyr invited comparison of the King to Jesus.
The pathos of this dramatic presentation made it a master stroke of Royalist propaganda. The book was quite popular despite official disapproval during the Protectorate and the Restoration; it went into 36 editions in 1649 alone. Because of the favourable impression the book made of the King, Parliament commissioned John Milton to write a riposte to it, which he published under the title Eikonoklastes ("The Icon-Breaker") in 1649. Milton's response sought to portray the image of Charles, and the absolute monarchy he aspired to, as idols, claiming a reverence due only to God, and therefore justly overthrown to preserve the law of God. This theological counterattack failed to dislodge the sentimental narrative of the Eikon itself from public esteem.
The Eikon Basilike and its portrait of Charles's execution as a martyrdom were so successful that, at the Restoration, a special commemoration of the King on January 30 was added to the Book of Common Prayer, directing that the day be observed as an occasion for fasting and repentance. On May 19, 1660, the Convocation of Canterbury and York canonised King Charles at the urging of Charles II, and added his name to the prayer book. Charles I is the only saint formally canonised by the Church of England.
The commemoration was removed from the prayer book by Queen Victoria in 1859. Several Anglican churches and chapels are dedicated to "King Charles the Martyr." The Society of King Charles the Martyr was established in 1894 to work for the restoration of the King's name to the Calendar and to encourage the veneration of the Royal Martyr.