16. When I heard . . . trembled--namely, at the judgments which God had declared (Hab 1:1-17) were to be inflicted on Judea by the Chaldeans.
      belly--The bowels were thought by the Hebrews to be the seat of yearning compassion (Jer 31:20). Or "heard" may refer to Hab 3:2, "When I heard as to Jehovah's coming interposition for Israel against the Chaldeans being still at some distance" (Hab 2:3); so also the voice" [MAURER].
      at the voice--of the divine threatenings (Hab 1:6). The faithful tremble at the voice alone of God before He inflicts punishment. Habakkuk speaks in the person of all the faithful in Israel.
      trembled in myself--that is, I trembled all over [GROTIUS].
      that I might rest in the day of trouble--The true and only path to rest is through such fear. Whoever is securely torpid and hardened towards God, will be tumultuously agitated in the day of affliction, and so will bring on himself a worse destruction; but he who in time meets God's wrath and trembles at His threats, prepares the best rest for himself in the day of affliction [CALVIN]. HENDERSON translates, "Yet I shall have rest." Habakkuk thus consoling his mind, Though trembling at the calamity coming, yet I shall have rest in God (Isa 26:3). But that sentiment does not seem to be directly asserted till Hab 3:17, as the words following at the close of this verse imply.
      when he cometh up unto the people, he will invade--rather (as English Version is a mere truism), connected with the preceding clause, "that I might rest . . . when he (the Chaldean foe) cometh up unto the people (the Jews), that he may cut them off" [CALVIN]. The Hebrew for "invade" means, to rush upon, or to attack and cut off with congregated troops.

JFB.