The New Testament book of Hebrews was written to a group of Christians who were perilously close to “departing from the living God” by developing in themselves “an evil heart of unbelief” (Heb. 3:12). The inspired writer was trying mightily to draw them back from the precipice of “destruction/perdition/ ruin” (10:39). Chapter six contains one of the serious warnings he gives.
For our 
purposes here, concentrate on verse six. Verses four and five beautifully depict the transformation one experiences in becoming a Christian. So, when you get to verse six, God is speaking of those who are already Christians (i.e., those who are saved). The KJV and NKJV begin the verse with the word “if” — “if they fall away.” At the very least, that word would indicate that apostasy is certainly possible, otherwise God would never speak of it. However, the ASV and NASB more accurately capture the meaning of the Greek verb, referring to those who were once Christians and “then fell (or ‘have fallen’) away.” This verb not only indicates that Christians can fall away but that they had (in reality) fallen away. A possibility and a reality!
May each of us “take heed lest [we] fall” (1 Cor. 10:12)!