Amos 1-4: The Lion Has Roared.

The prophet Amos was only a Shepherd, but he heard the words of the Lord.  Over and over again, God revealed his frustration and anger with his chosen people through the prophets.  It wasn't as if there was no warning to Israel that their sins, of both commission and omission, were taking them to the point of no return.
   Surely the Sovereign LORD does nothing
       without revealing his plan
       to his servants the prophets.
   The lion has roared—
       who will not fear?
       The Sovereign LORD has spoken—
       who can but prophesy?  - Amos 3:7-9 NIV

God was fed up with their lip service when their actions belied what was in their hearts.
   "Go to Bethel and sin;
       go to Gilgal and sin yet more.
       Bring your sacrifices every morning,
       your tithes every three years.
  Burn leavened bread as a thank offering
       and brag about your freewill offerings—
       boast about them, you Israelites,
       for this is what you love to do,"
       declares the Sovereign LORD.  - Amos 4:4-5 NIV

God saw Israel's heart.  He had warned them over and over again with signs of his anger at them: drought, plagues, overthrown towns and villages, yet they still had not returned to him.  All the dire warnings of the prophets will be fulfilled.
  "Therefore this is what I will do to you, Israel,
       and because I will do this to you,
       prepare to meet your God, O Israel."
   He who forms the mountains,
       creates the wind,
       and reveals his thoughts to man,
       he who turns dawn to darkness,
       and treads the high places of the earth—
       the LORD God Almighty is his name. - Amos 4:12-13 NIV

The Lion has roared...prepare to meet your God.