Therefore strong peoples will glorify you; cities of ruthless nations will fear you. (Isaiah 25:3)
Isaiah sees the day coming when all the nations — representatives from all the people groups — will no longer be at odds with Yahweh, the God of Israel and his Messiah, whom we know to be Jesus.
They will no longer worship Bel or Nebo or Molech or Allah or Buddha or utopian social programs or capitalistic growth possibilities or ancestors or animistic spirits. Instead they will come in faith to the banquet on God’s mountain. And they will have the veil of sorrow removed and death shall be swallowed up and the reproach of God's people will be removed and tears shall be gone forever.
That's the setting for understanding the vision of verse 3: “Therefore strong peoples will glorify you; cities of ruthless nations will fear you.” In other words God is stronger than the “strong people” and he is so powerful and so gracious that in the end he will turn ruthless nations to revere him.
So the picture Isaiah gives us is one of all nations turned to God in worship, a great banquet for all the peoples, the removal of all suffering and grief and reproach from the nations who have become his people, and the final putting away of death forever.
This triumph is sure because God is doing it. Therefore we can be certain of it.
Not one life spent in the cause of world evangelization is spent in vain. Not one prayer or one dollar or one sermon or one letter of encouragement mailed or one little light shining in some dark place — nothing in the cause of the advancing kingdom is in vain.
The triumph is sure.