It is almost impossible to find an instance in which Knox allows that he may have been mistaken: d'avoir toujours raison was his claim.

If he admits an error in details, it is usually an error of insufficient severity.

He did not attack Northumberland or Mary Stuart with adequate violence; he did not disapprove enough of our prayer book; he did not hand a heretic over to the magistrates.
While acting as a priest and notary, between 1540, at latest, and 1543, Knox was engaged as private tutor to a boy named Brounefield, son of Brounefield of Greenlaw, and to other lads, spoken of as his "bairns." In this profession of tutor he continued till 1547.
Knox's personal aspect did not give signs of the uncommon strength which his unceasing labours demanded, but, like many men of energy, he had a perpetual youth of character and vigour.

After his death, Peter Young described him as he appeared in his later years.

He was somewhat below the "just" standard of height; his limbs were well and elegantly shaped; his shoulders broad, his fingers rather long, his head small, his hair black, his face somewhat swarthy, and not unpleasant to behold.