What does Malachi 3:17 mean?
In this context, "the day" is a reference to the time of ultimate judgment. This book of prophecy began by showing how God treated Edom differently from Israel (Malachi 1:2–5). Those God chose as His people were kept safe from the obliteration experienced by those who defied Him. This particular promise is tied more to repentance than to the nation of Israel, making it an interesting segue into the gospel which will come some four centuries later. These words are among the last to be delivered to Israel before this period of silence, ended by John the Baptist.Also noteworthy is the reference to being spared "as a man spares his son." Part of the promise of the gospel is adoption by God (Galatians 4:5; Ephesians 1:5). While this particular mention is not an explicit reference to salvation, it hints at a theme that later Scriptures will clarify.
Verse 18 will explain part of the purpose behind this rescue: to show the world the difference between those who belong to God and those who reject Him.