Barnes New Testament Notes

Verse 18. But it is good to be zealously affected. The meaning of this is, "Understand me: I do not speak against zeal. I have not a word to say in its disparagement. In itself, it is good; and their zeal would be good if it were in a good cause." Probably, they relied much on their zeal; perhaps they maintained, as errorists and deceivers are very apt to do, that zeal was sufficient evidence of the goodness of their cause, and that persons who are so very zealous could not possibly be bad men. How often is this plea set up by the friends of errorists and deceivers!

And not only when I am present with you. It seems to me that there is great adroitness and great delicacy of irony in this remark; and that the apostle intends to remind them, as gently as possible, that it would have been as well for them to have shown their zeal in a good cause, when he was absent, as well as when he was with them. The sense may be, "You were exceedingly zealous in a good cause when I was with you. You loved the truth; you loved me. Since I left you, and as soon almost as I was out of your sight, your zeal died away, and your ardent love for me was transferred to others. Suffer me to remind you, that it would be well to be zealous of good when I am away, as well as when I am with you. There is not much true affection in that which dies away as soon as a man's back is turned." The doctrine is, that true zeal or love will live alike when the object is near, and when it is removed; when our friends are present with us, and when they leave us; when their eye is upon us, and when it is turned away.

{a} "always in a good thing" 1 Co 15:58