John Chrysostom

"Not backward in zeal." For this also gendereth love when with honor we also show a readiness to protect: as there is nothing that makes men beloved so much as honor and forethought. For to love is not enough, but there must be this also: or rather this also comes of loving, as also loving has its warmth from this, and they are confirmative one of another. For there are many that love in mind, yet reach not forth the hand. And this is why he uses every means to build up love. …

Homily on Romans 21 ↗

Theophylact of Ohrid

Since many, it would seem, honor and love one another in their souls, but do not extend their hands to them, he teaches us to care for one another and to help others by deed itself. An objection is anticipated: how can we "not be slothful in zeal"? If, he answers, we are fervent in spirit, that is, if we are diligent and zealous. Note also that "be fervent in spirit" is said after the enumeration of many kinds of virtue; for whoever has fulfilled the above attracts the grace of the Spirit and is kindled by His fire. …

Commentary on Romans ↗

Thomas Aquinas

Then when he says, in carefulness, he shows the qualities surrounding charity's love of God. First, he starts with reason's attentiveness when he says: in carefulness, not slothful, when it is a question of worshiping God: he has shown you, O man, what is good; and what does the Lord require of you, and he continues: but to walk carefully with your God? (Mic 6:8); do your best to present yourself to God as one approved (2 Tim 2:15). Second, the affections when he says: be fervent in the spirit, namely, in loving God. …

Commentary on Romans ↗