Do not consider yourself to have made any spiritual progress unless you account yourself the least of all men.

[Non reputes te aliquid profecisse, nisi omnibus te inferiorem esse sentias.]

Thomas à Kempis (c. 1380-1471) German-Dutch priest, author

The Imitation of Christ [De Imitatione Christi], Book 2, ch. 2, v. 4 (2.2.4) (c. 1418-27) [tr. Sherley-Price (1952)]
    (Source)

(Source (Latin)). Alternate translations:

Moreover, if thou wilt come to the highness of perfection, think not thyself to have profited anything in virtue, till thou canst feel meekly in thy heart that thou hast less meekness and less virtue than hath any other.
[tr. Whitford/Raynal (1530/1871)]

Moreover, if you will come to the height of perfection, do not think that you have advanced in virtue until you can feel humbly in your heart that you have less humility and less virtue than anyone else.
[tr. Whitford/Gardiner (1530/1955)]

Think not that thou hast profited any thing at all, unlesse thou hast learnt to esteem thy selfe inferiour unto all.
[tr. Page (1639), 2.2.11]

No Man ought to esteem himself a Proficient in Goodness, who is not yet arrived to that Pitch of it, which teaches him to think himself the least of all Saints and last of all Men.
[tr. Stanhope (1696; 1706 ed.)]

Do not think that thou hast made any progress towards perfection, till thou feelest, that thou art less than the least of all human beings.
[tr. Payne (1803)]

Do not think that thou hast made any progress, unless thou esteem thyself inferior to all.
[ed. Parker (1841)]

Do not think that thou hast made any progress towards perfection, till thou feelest that thou art "less than the least of all" human beings.
[tr. Dibdin (1851)]

Never think thou hast made any progress till thou feel thyself inferior to all.
[ed. Bagster (1860)]

Reckon not thyself to have profited in anywise unless thou feel thyself to be inferior to all.
[tr. Benham (1874)]

Do not think that thou hast made any progress, unless thou esteem thyself inferior to all.
[tr. Anon. (1901)]

Hence, you must not think that you have made any progress until you look upon yourself as inferior to all others.
[tr. Croft/Bolton (1940)]

Consider yourself not to have made any progress unless you feel yourself inferior to all men.
[tr. Daplyn (1952)]

Never think that you have made any progress, till you have learned to regard all men as your betters.
[tr. Knox-Oakley (1959)]

Only when you think yourself of less importance than everybody else may you consider that you have made some progress.
[tr. Knott (1962)]

Consider that you have made no progress at all until you recognize that you are lower than everybody else.
[tr. Rooney (1979)]

Do not think you have made any progress unless you feel truly humble before God and others.
[tr. Creasy (1989)]