THEOparadox Advisor Tony Hayling has been doing a wonderful series on the book of Hebrews at his site, Agonizomai. Below, I've extracted a quote in which Tony points out the great paradox of chapter 4, where we are given a dual exhortation to "rest" and to "strive."
"After having established that no one can enter God’s rest apart from being found obedient through faith, the writer immediately exhorts the Hebrews to strive to enter that rest! What can striving and resting have to do with each other? And this is the wonder of the gospel. To the fallen mind it is full of apparent contradiction. To the regenerate person it is full of truth and light. We may not be able, as saints, to reconcile in our minds all of God’s antinomies (a J.I. Packer term) - but we are willing to believe that they can be reconciled and that, on that Day, they shall be. So the person of faith does not know everything. On the contrary, he knows that he knows nothing - and it is this humility of mind that enables him to receive what he previously would not, and to strive to be found under the authority and guidance of God."
Note: Recipients of our t-shirts usually don't get arms, but in this case I've added prosthetics so Tony can operate his sound equipment. He records almost all of his blog posts and makes them available using a handy media player embedded on the site.