Striving for Our Rest (Hebrews 3:7-4:13 (LD 22; BC 37)

We are exhorted to strive to enter the Sabbath rest of the Lord.  Does this mean that we have to work really hard to please God?  Was the problem with Israel that they just did not understand the gospel?  Was it that they did not have Christ?  Why would the letter to the Hebrews appeal to Israel when we are being exhorted through the words of this letter?  What can we possibly learn from the Israelites that is relevant today?