Hebrews 6 is one of the key texts used to defend the biblical doctrine of apostasy. Apostasy refers to the rejection of the Gospel by those who had previously known it. However, the real tough question that has been asked over the years is whether or not apostasy has to do with genuinely converted sons (daughters) of God or unconverted outwardly religious folk. I believe Hebrews 6:4-6 can help us to answer that question.
So, to answer the initial question: were the apostates of Hebrews 6 true believers? No. Not at all. An apostate may be enlightened intellectually, taste the Gospel superficially, partake in an external manner in the work of the Spirit and know something of the glory of the economy of the Gospel, but he (she) has not been transformed from within by the power of the God.