Nineteenth-century NT scholar B. F Westcott, commenting on Heb. 2:18, writes:
Sympathy with the sinner in his trial does not depend on the experience of sin but on the experience of the strength of the temptation to sin which only the sinless can know in its full intensity. He who falls yields before the last strain.--Brooke Foss Westcott, The Epistle to the Hebrews (1892), 59
In a 1943 letter, C. S. Lewis alludes to this comment by Westcott, and it seems that Westcott was the one to influence Lewis' own similar but more well-known statement on temptation in Mere Christianity.