His warlike renown endeared him to the king, and Prince Henry counted him among the most illustrious of his servants.

The favour of the royal house was the more noteable that Oldcastle was known as "leader and captain" of the Lollards.

His Kentish castle of Cowling served as the headquarters of the sect, and their preachers were openly entertained at his houses in London or on the Welsh border.

The Convocation of 1413 charged him with being "the principal receiver, favourer, protector, and defender of them; and that, especially in the dioceses of London, Rochester, and Hereford, he hath sent out the said Lollards to preach ...
and hath been present at their wicked sermons, grievously punishing with threatenings, terror, and the power of the secular sword such as did withstand them, alleging and affirming among other matters that we, the bishops, had no power to make any such Constitutions" as the Provincial Constitutions, in which they had forbidden the preaching of unlicensed preachers.

The bold stand of Lord Cobham drew fresh influence from the sanctity of his life.