John Knox was a Scottish minister, Reformed theologian, and writer who was a leader of the country's Reformation. He was the reformer of the Church of Scotland 1514-1572.
Measurements are approx. 28 1/2" x 38 1/2"
Given the canvas support and the way the paint has aged, this is very unlikely to be any earlier than 17th Century so at any rate, it is a posthumous portrait.
Purchased at a Baker auction here in Clackamas, Or. in 1968. The auction catalog listed the oil canvas painting as a Portrait of John Knox