Therefore let us be grateful for receiving a kingdom that cannot be shaken, and thus let us offer to God acceptable worship, with reverence and awe, for our God is a consuming fire. - Hebrews 12:28-29 

Calvin: “By saying that God is to be served acceptably…with reverence and fear, he intimates that though he requires us to serve with promptitude and delight, there is yet no service approved by Him except it be united with humility and due reverence…

As he had before kindly set before us the grace of God, so he now makes known his severity…Thus we see that God omits nothing by which he may draw us to himself; he begins indeed with love and kindness, so that we may follow him the more willingly, but when by alluring he effects but little, He terrifies us…

And doubtless it is expedient that the grace of God should never be promised to us without being accompanied with threatenings; for we are so extremely prone to indulge ourselves, that without the application of these stimulants the milder doctrine would prove ineffectual.  Then the Lord, as he is propitious and merciful to such as fear him unto a thousand generations; so He is a jealous God and a just avenger when despised, unto the third and fourth generation.”