Colloquium: The Australian and New Zealand Theological Review
The difficulty that Hebrews 6:1-8 presents to those in the Calvinist tradition is that these verses seem to teach - contrary to a strict doctrine of election and perseverance - that even true believers (v v 4-5) can fall away (parapesontas, v 6). For those who are not within the Calvinist tradition, the problem of these verses is not the apparent teaching that sincere Christians can fall away, but the statement that it is impossible (adunation, v 4) to renew apostates unto repentance (palin anakainizein eis metanoian, v 6).
Young, N. (1982). Is Hebrews 6:1-8 Pastoral nonsense? Colloquium: The Australian and New Zealand Theological Review, 15(1), 52-57.