I don't believe once saved always saved. However Hebrews Ch.6 vs.6 sounds like once fallen away, you can never return. I don't think that is the case, but after reading that scripture, how else would you explain it?
The context of the passage, going back to verses 1-3 of the chapter, is laying aside discussion of basics and going on to growth in deeper matters. Verses 4-6 go on to say that if one is repeatedly taught these basics (repentance, faith, baptism, etc.) and later rejects them, it will be impossible to reach him again with the same arguments. He has rejected these arguments as a basis for his faith, so it will take different or stronger arguments to bring him back to faith. If, however, his teachers go beyond the basics when he is able to take in deeper matters, then there is less likelihood that he will fall away. The writer goes on to a discussion of deeper matters (the priesthood of Christ) that would make the good news brought by the basics that much more intense. An understanding of these deeper matters serves to strengthen one's faith and joy in salvation.
In short, I take the "impossible" here to mean impossible to renew him to repentance with the same arguments.