To be religious is to serve God. How? (1.) We must make a business of it, and not be slothful in that business. Not slothful in business. There is the business of the world, that of our particular calling, in which we must not be slothful, 1 Thess. iv. 11. But this seems to be meant of the business of serving the Lord, our Father's business, Luke ii. 49. Those that would approve themselves Christians indeed must make religion their business—must choose it, and learn it, and give themselves to it; they must love it, and employ themselves in it, and abide by it, as their great and main business. And, having made it our business, we must not be slothful in it: not desire our own ease, and consult that, when it comes in competition with our duty. We must not drive on slowly in religion. Slothful servants will be reckoned with us wicked servants. (2.) We must be fervent in spirit, serving the Lord. God must be served with the spirit (ch. i. 9; John iv. 24), under the influences of the Holy Spirit. Whatever we do in religion it is pleasing to God no further than it is done with our spirits wrought upon by the Spirit of God. And there must be fervency in the spirit—a holy zeal, and warmth, and ardency of affection in all we do, as those that love God not only with the heart and soul, but with all our hearts, and with all our souls. This is the holy fire that kindles the sacrifice, and carries it up to heaven, an offering of a sweet-smelling savour.

Matthew Henry on Rom 12:11