William Burkitt on Galatians 4:18

Note here, That although there is a zeal in a bad cause, which is to be condemned and avoided, yet there is a zeal in a just and righteous cause, which is so laudable and worthy to be practised; when it is a zeal guided by religion, governed by prudence, attended with perseverance; when in a good thing we are affected, zealously affected, and zealously affected always. This the apostle desires that the Galatians should be, and that as well when he was absent from them, as when he was present with them.