Answer:
This was from one of my commentaries and explains this verse well.
Good question – you must study the scripture.
Mal 3:16
16. “Then,” when the ungodly utter such blasphemies against God, the godly hold mutual converse, defending God’s righteous dealings against those blasphemers (Heb 3:13). The “often” of English Version is not in the Hebrew. There has been always in the darkest times a remnant that feared God (1Ki 19:18; Ro 11:4).
feared the Lord–reverential and loving fear, not slavish terror. When the fire of religion burns low, true believers should draw the nearer together, to keep the holy flame alive. Coals separated soon go out.
book of remembrance . . . for them–for their advantage, against the day when those found faithful among the faithless shall receive their final reward. The kings of Persia kept a record of those who had rendered services to the king, that they might be suitably rewarded (Es 6:1-2; compare Es 2:23; Ezr 4:15; Ps 56:8; Isa 65:6; Da 7:10; Re 20:12). CALVIN makes the fearers of God to be those awakened from among the ungodly mass (before described) to true repentance; the writing of the book thus will imply that some were reclaimable among the blasphemers,