For man proposes but God disposes. The path a person takes does not lie within himself.

[Nam homo proponit, sed Deus disponit, nec est in homine via ejus.]

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

The Imitation of Christ [De Imitatione Christi], Book 1, ch. 19, v. 2 (1.19.2) (c. 1418-27) [tr. Creasy (1989)]
    (Source)

Thomas saying that, regardless of a person's good intentions to act virtuously, they are dependent on God's grace to make that actually happen.

The phrase "Man proposes but God disposes" (or the Latin original of it) was coined by Thomas, which makes it ironic where some later translators put it in quotations or self-referent indeeds.

The text given relates to, is frequently footnoted to, and even is quoted directly from:

• Proverbs 16:9 ("A man's heart deviseth his way: but the Lord directeth his steps." [KJV])
• Jeremiah 10:23 ("O Lord, I know that the way of man is not in himself: it is not in man that walketh to direct his steps." [KJV])

(Source (Latin)). Alternate translations:

For man purposeth, but God disposeth: nay, the way that man shall walk in this world is not in himself but in the grace of God.
[tr. Whitford/Raynal (1530/1871)]

Man proposes, but God disposes. The way that a man shall walk in this world is found not in himself, but in the grace of God.
[tr. Whitford/Gardiner (1530/1955)]

For man doth propose but God doth dispose, neither is the way of man in his owne hands.
[tr. Page (1639), 1.19.9]

A Man's Heart deviseth his Way, but the Lord directeth his Steps, says Solomon: We may contrive and act as seems most adviseable; by which we do so, are from the Lord, so is the Event of our having done it entirely in his disposal.
[tr. Stanhope (1696; 1706 ed.), 1.19.3]

Tho' the heart of man deviseth his way, yet the Lord ordereth the event; and that it is not in man that walketh, to direct his steps.
[tr. Payne (1803)]

For man proposes, but God disposes; neither is the way of man in himself.
[Parker ed. (1841); Bagster ed. (1860); Anon. (1901)]

For man proposes but GOD disposes: nor is it in man to direct his steps.
[tr. Dibdin (1851)]

For man proposeth, but God disposeth; and the way of a man is not in himself.
[tr. Benham (1874)]

For man, indeed, proposes but God disposes, and God's way is not man's.
[tr. Croft/Bolton (1940)]

For man proposes, but God disposes, and a man's road is not within himself.
[tr. Daplyn (1952)]

Man proposes, but God disposes, and man's destiny is not in his own hands.
[tr. Sherley-Price (1952)]

They know that "man proposes, and God disposes"; the course of a man's life is not what he makes it.
[tr. Knox-Oakley (1959)]

For man proposes, God disposes, and it is not for man to choose his lot.
[tr. Knott (1962)]

Man indeed proposes, bit it is God who disposes nor is the course of man in his power as he goes his way.
[tr. Rooney (1979)]