In context the author of Hebrews (some believe the book was written by the apostle Paul but this is uncertain) is stressing the superiority of Christ to the angels. He cites Psalm 8 to reflect on how Christ “was made a little lower than the angels for the suffering of death, crowned with glory and honour” (v. 9). Though confident that one day all things will plainly be made subject to Christ, the author also acknowledges that this has not yet come about. “But now we see not yet all things put under him.” Theologians sometime speak of the “already” and the “not yet” of the Christian life. This little verse captures that truth.
Now our local assemblies are mainly weak little flocks. We seek a regenerate community, but we are often little better than a mixed multitude. Tares are sown among the wheat. But one day we will join the righteous myriads in heaven of just men made perfect through the blood of Christ.
Take one look at the world around you, and it will be quite plain that this world is not yet all it will one day will be. That’s not a bad thing to keep in mind throughout your day: “But now we see not yet all things put under him.”