malachi 3:6 For I am the LORD, I change not; therefore ye sons of Jacob are not consumed. - Free Bible Online

      6. the Lord--Jehovah: a name implying His immutable faithfulness in fulfilling His promises: the covenant name of God to the Jews (Ex 6:3), called here "the sons of Jacob," in reference to God's covenant with that patriarch.
      I change not--Ye are mistaken in inferring that, because I have not yet executed judgment on the wicked, I am changed from what I once was, namely, a God of judgment.
      therefore ye . . . are not consumed--Ye yourselves being "not consumed," as ye have long ago deserved, are a signal proof of My unchangeableness. Ro 11:29: compare the whole chapter, in which God's mercy in store for Israel is made wholly to flow from God's unchanging faithfulness to His own covenant of love. So here, as is implied by the phrase "sons of Jacob" (Ge 28:13; 35:12). They are spared because I am JEHOVAH, and they sons of Jacob; while I spare them, I will also punish them; and while I punish them, I will not wholly consume them. The unchangeableness of God is the sheet-anchor of the Church. The perseverance of the saints is guaranteed, not by their unchangeable love to God, but by His unchangeable love to them, and His eternal purpose and promise in ChristJesus [MOORE]. He upbraids their ingratitude that they turn His very long-suffering (La 3:22) into a ground for skeptical denial of His coming as a Judge at all (Ps 50:1, 3, 4, 21; Ec 8:11, 12; Isa 57:11; Ro 2:4-10).

JFB.