Therefore introduces the practical result of the chapter; see comments on 1.7, where the same Greek word is translated so that.

My beloved brethren is a particularly affectionate phrase used by Paul only here and in Phil 4.1. The phrase also has the function of calling special attention to the words that follow. Brethren, as usual, means “fellow Christians.”

The words abounding, your labor, and in vain recall verse 10. See the comments.

The clause in the Lord your labor is not in vain means “your work is not useless when you do it as Christians,” or even “the Lord [Christ] will make sure that your work does not prove useless.” The words in the Lord are strongly emphasized in the Greek by being at the end of the sentence.

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .