“Do not be slothful in zeal, be fervent in spirit, serve the Lord.” -Romans 12:11
This is an interesting statement, primarily because although the words are familiar to us, the construction of his language is fairly uncommon. The first thing one might notice is the trifecta of words that all have to do with one’s enthusiasm: not slothful, zeal, and fervent. Slothful is the opposite of zeal and fervency, so Paul says to not be slothful. But the exhortation to avoid slothfulness seems to be aimed at a person’s zeal. The first question, then, is what is zeal? Following up, Paul says, in contrast to his exhortation to avoid sloth, be fervent in spirit. Finally, serve the Lord.
In zeal is from τῇ σπουδῇ (spoude) meaning the earnest commitment in discharge of an obligation. The obligation in this case is our spiritual worship, our service to the Lord as a reasonable response to his abundant mercies (Romans 12:1,2).
In other words, Paul is exhorting believers, the new humanity that has been born again by the Spirit of God, to not be lazy in our service to the Lord. Instead, we are to be fervent (enthusiastic) in spirit about serving God and discharging our worship.