The inclusion of this exclamatory statement is rather typical of Paul (see 1 Thes 2.5; 2 Cor 1.23; 11.31).

What I write can refer either to what immediately precedes or to all that precedes. For the former, see New American Bible “What I have just written,” also Jerusalem Bible; for the latter, see Phillips “All this that I am telling you.”

In order to avoid possible contradiction with the interpretation of 6.11, in which Paul speaks of writing with his own hand, it may be useful in verse 20 to say “what I am telling you.” In this way one can avoid the impression that Paul wrote the entire letter personally rather than dictating it, as it is usually understood that he did.

God knows that I am not lying functions here as an oath (compare Knox “as God sees me,” Moffatt “I swear it before God,” Phillips “I assure you before God”). What Paul is trying to emphasize is the fact that he is telling the truth. The very fact that he can affirm with an oath that he is telling the truth would indicate to his readers that he is indeed not lying. He can risk a curse (implied by the oath) for he knows God knows that he is not lying. An equivalent expression in some languages is “God can tell you that I am telling the truth.”

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 .