What does 'eternal' mean in Ephesians 3:11?

In Ephesians 3:11, 'eternal' refers to eternity past—before anything was created, before heaven, the first angel, or any celestial body existed. It describes God's purpose settled in that eternal past, when He purposed for the human race in Christ Jesus our Lord.

Les explains that when Ephesians 3:11 speaks of “according to the eternal purpose,” the word eternal reaches all the way back to eternity past. That is before there was ever anything created—before heaven, before the first angel, before the first star or celestial body. From that eternity past, God already had His purposes for the human race, purposed in Christ Jesus our Lord. The thought flows from the manifold wisdom of God in verse 10 right into verse 11.

Ephesians 3:11

According to the eternal purpose which he purposed in Christ Jesus our Lord:

What does the word 'eternal' in 'eternal purpose' refer back to?

The word 'eternal' refers back to eternity past—before anything was created, before heaven, the first angel, or the first star and celestial body. From that eternity past, God purposed His plans for the human race in Christ Jesus our Lord, so the purpose is rooted in His pre-creation counsel.