How can you be so foolish expresses unbelief on the part of Paul. He finds it difficult to believe that the Galatians can be that foolish (compare Phillips “Surely you can’t be so idiotic”; New English Bible “Can it be that you are so stupid?”). In place of an exclamatory question How can you be so foolish!, it may be necessary in some languages to use a strong negative expression, for example, “You surely cannot be so foolish!” or, idiomatically, “Certainly your minds must not have left you so completely!”
You began by God’s Spirit; do you now want to finish by your own power? is literally “Having begun with (in, by) the Spirit, with (in, by) the flesh are you now finishing?” Here Paul presents a twofold contrast: beginning/finishing, God’s Spirit/flesh. As in the previous verse, “Spirit” here again refers to the Spirit of God. “Flesh” could be interpreted in many ways. It could mean the “body,” that is, a reference to what is circumcised. Or it could refer to the natural powers of men apart from the divine Spirit, hence the Good News Translation rendering by your own power. Again, “flesh” could refer to outward observances such as the Jewish rite of circumcision and other requirements of the Law (Jerusalem Bible “Are you foolish enough to end in outward observances what you began in the Spirit?”; Phillips “reverting to outward observances”; but see New English Bible, where the antithesis is between the “material” and the “spiritual”: “You started with the spiritual; do you now look to the material…?”).
One of the serious complications involved in this contrast between God’s Spirit and a person’s own power is the fact that the verb began occurs without any verbal complement. In a number of languages one simply cannot use a verb such as began without indicating what began. In some languages one can say “You began your new life by means of God’s Spirit,” or “God’s Spirit caused you to begin to live in a new way.” It is also possible to speak of “your new relation to God.”
In place of the question do you now want to finish by your own power?, one may employ a statement for languages which would not use a rhetorical question. One may say, for example, “you certainly do not want to complete your life by just what you can do for yourself.” However, one may wish to restructure the relations rather extensively, for example, “how do you think that you are strong enough to complete the life that you have begun?”, or “how do you think that you can continue living to the end by your own strength?”
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 .