3. Call . . . I will answer-- (Jer 29:12; Ps 91:15). Jeremiah, as the representative of the people of God, is urged by God to pray for that which God has determined to grant; namely, the restoration. God's promises are not to slacken, but to quicken the prayers of His people (Ps 132:13, 17; Isa 62:6, 7).
      mighty things--Hebrew, "inaccessible things," that is, incredible, hard to man's understanding [MAURER], namely, the restoration of the Jews, an event despaired of. "Hidden," or "recondite" [PISCATOR].
      thou knowest not--Yet God had revealed those things to Jeremiah, but the unbelief of the people in rejecting the grace of God had caused him to forget God's promise, as though the case of the people admitted of no remedy.

JFB.