Mal. 3:16 and 17 refers primarily to the godly remnant in Israel at the close of the old dispensation. All was ruin and apostasy around them; but they feared the Lord, thought upon His name, and spoke often one to another. They did not attempt to set up anything, or reconstruct a fallen system; they owned the ruin, feared the Lord, and communed one with another.
Jude 20-23 gives us a Christian remnant in the midst of the ruin of professing Christianity. You will find it interesting and instructive to mark the points of similarity and of contrast in these two passages of Scripture.