Hebrews 12:28-29
“Wherefore we receiving a kingdom which cannot be moved, let us have grace [or gratitude,] whereby we serve God acceptably with reverence and godly fear. For our God is a consuming fire.”
1. In another place he says the same, “for the things which are seen are temporal, but the things which are not seen are eternal”; and from this makes an exhortation with regard to the evils which we endure in this present life; and here he does this, and says, let us continue steadfast; “let us have thankfulness,” i.e., let us give thanks unto God. For not only we ought not to be discouraged at present things, but even to show the greatest gratitude to Him, for those to come.
“Whereby we serve God acceptably,” that is to say, 'for thus is it possible to serve God acceptably,' by giving him thanks in all things. “Do all things” (he says) “without murmurings and disputings.” For whatever work a man does with murmuring, he cuts away and loses his reward; as the Israelites— how great a penalty they paid for their murmurings. Wherefore he says, “Neither murmur ye.” It is not therefore possible to “serve” Him “acceptably” without a sense of gratitude to Him for all things, both for our trials, and the alleviations of them. That is, let us utter nothing hasty, nothing disrespectful, but let us humble ourselves that we may be reverential. For this is “with reverence and godly fear.”
Source: Homilies on the Epistle to the Hebrews (New Advent)