Sir John Oldcastle (died 14 December 1417) was an English Lollard leader. From 1409 to 1413, he was summoned to parliament as Baron Cobham, in the right of his wife.
Being a friend of Henry V, he long escaped prosecution for heresy. When convicted, he escaped from the Tower of London and then led a rebellion against the King. Eventually, he was captured and executed in London. He formed the basis (alongside Sir John Fastolf) for William Shakespeare's character John Falstaff, who was originally called John Oldcastle.
Oldcastle's date of birth is unknown; dubious and possibly apocryphal sources place it variously at 1360 or 1378, although 1370 is most likely closer to the truth.
His parents were Richard and Isabel Oldcastle of Almeley, Herefordshire. His grandfather, also called John Oldcastle, was Herefordshire's MP during the latter part of the reign of King Richard II.
Oldcastle is first mentioned in two separate documents in 1400, first as a plaintiff in a suit regarding the advowson of Almeley church, and again as serving as a knight under Lord Grey of Codnor in a military expedition to Scotland.
Over the next few years, Oldcastle held notable positions in the Welsh campaigns of King Henry IV of England against Owain Glyndŵr, including captaincy first over Builth Castle in Brecknockshire and then over Kidwelly.
Oldcastle represented Herefordshire as a "knight of the shire" in the parliament of 1404, later serving as a justice of the peace, and was Sheriff of Herefordshire in 1406–07.
In 1408, he married Joan, the heiress of Cobham — his third marriage, and her fourth. This resulted in a significant improvement of his fortune and status, as the Cobhams were "one of the most notable families of Kent". The marriage brought Oldcastle a number of manors in Kent, Norfolk, Northamptonshire and Wiltshire, as well as Cooling Castle, and from 1409 until his accusation in 1413 he was summoned to parliament as Lord Cobham.
At some point in his military career, Oldcastle became a trusted supporter of Henry, Prince of Wales, later to become King Henry V, who regarded Sir John as "one of his most trustworthy soldiers".
Oldcastle was a member of the expedition which the young Henry sent to France in 1411 in a successful campaign to assist the Burgundians in the Armagnac-Burgundian Civil War.
Lollardy had many supporters in Herefordshire, and Oldcastle himself had adopted Lollard doctrines before 1410, when the churches on his wife's estates in Kent were laid under interdict for unlicensed preaching. In the convocation which met in March 1413, shortly before the death of Henry IV, Oldcastle was at once accused of heresy.
But his friendship with the new King Henry V prevented any decisive action until convincing evidence was found in one of Oldcastle's books, which was discovered in a shop in Paternoster Row, London. The matter was brought before the King, who desired that nothing should be done until he had tried his personal influence. Oldcastle declared his readiness to submit to the king "all his fortune in this world" but was firm in his religious beliefs.
When Oldcastle fled from Windsor Castle to his own castle at Cooling, Henry at last consented to a prosecution. Oldcastle refused to obey archbishop Thomas Arundel's repeated citations, and it was only under a Royal Writ that he at last appeared before the ecclesiastical court on 23 September 1413.
In a confession of his faith, Oldcastle declared his belief in the sacraments and the necessity of penance and confession addressed to God, but he would not assent to the orthodox doctrine of the sacraments as stated by the Bishops, nor admit the necessity for the Sacrament of Confession and Absolution being administered by a priest. He also said, similarly to the Iconoclasts of the Byzantine Empire and in contradiction of St John of Damascus, that the use of Christian art as an aid to prayer is, "the great sin of idolatry". On 25 September, Oldcastle was convicted as a heretic and sentenced to execution by burning.
King Henry V was still anxious to find a means of sparing the life of his old comrade, and ordered a reprieve of forty days in the hopes that Oldcastle might repent his views. Before that time had expired, however, Oldcastle had escaped from the Tower with the help of one William Fisher, a Lollard parchment-maker from Smithfield.
Oldcastle now put himself at the head of a widespread Lollard conspiracy. Despite the Christian pacifism preached by Wycliffe, the conspiracy were plotting regime change using violence and, if necessary, even civil war. The plan was to seize the King and his brothers in a palace coup during the Twelfth-night mumming at Eltham, and establish some sort of Commonwealth. Oldcastle would to be declared Regent, the king, English nobility and clergy would then be imprisoned. The religious orders would be forcibly dissolved and the property of the Catholic Church in England was to be divided up among the Lollards and used to create a new ruling class. King Henry, forewarned of Oldcastle's intentions by two moles inside the Lollard movement, instead moved to London. When the Lollard army assembled in force in St Giles's Fields on 10 January they were easily defeated by the king and the vastly superior numbers in his forces.
Oldcastle escaped into deepest northwest Herefordshire, and for nearly four years avoided capture.
He is believed to have been privy to the Southampton Plot in July 1415, when the Lollards pledged support and Oldcastle stirred some movement in the Welsh Marches. However, following the failure of the scheme he went again into hiding.