Hebrews 12:29

29.For our God, etc. As he had before kindly set before us the grace of God, so he now makes known his severity; and he seems to have borrowed this sentence from the Deuteronomy 4:24 of Deuteronomy. 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 threatening; 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 the fourth generation. (274)