Paul uses the expression do not deceive yourselves to introduce a general principle. The form of the Greek is passive (Revised Standard Version “do not be deceived”), and it is possible that Paul is thinking of the false teachers as those who would do the deceiving. It is better, however, to interpret this passive as being reflexive, as Good News Translation does. The general import of what Paul wants to convey is captured by such expressions as “make no mistake about it” (New American Bible, New English Bible, Knox), or “you can be doubly sure of this.”

In no one makes a fool of God, the word God is emphatic, emphasizing that God is not a man and therefore cannot be “mocked.” The Greek word for “mock” is related to the word “nose,” and can be translated literally as “to turn up the nose at.” Possible meanings are “to treat with contempt,” “to outwit,” “to cheat” (Jerusalem Bible, Knox). The sense of the whole statement is captured by Good News Translation (compare Phillips “you cannot make a fool of God!”; Jerusalem Bible “don’t delude yourself into thinking God can be cheated”). One may also simply translate “you cannot cheat God.”

If the proverbial saying is dropped in favor of nonfigurative language, what he plants could refer to anything that a man does while he is alive, and will reap could refer to God’s response to these actions or to his verdict in the day of judgment. An equivalent of this proverbial saying may be rendered in some languages as “Whatever a man plants in his field, that is exactly what he is going to harvest,” or “The kind of grain a man plants is the kind of grain he will harvest.”

Quoted with permission from Arichea, Daniel C. and Nida, Eugene A. A Handbook on Paul’s Letter to the Galatians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .