Dr. Lloyd-Jones examines the difficult passage in Hebrews 6 concerning salvation. This is an excellent and in-depth explanation of this hard-to-understand section of Hebrews.
For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
Hebrews 6:7-8 (ESV)