Can one be saved and then choose to turn away from God?

In this episode, Dr. Betters answers the question, "Is it, 'once saved always saved?' Or can one be saved and choose to turn away from God? Hebrews 6:4-6 says 'It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace. Land that drinks in the rain often falling on it and that produces a crop useful to those for whom it is farmed receives the blessing of God. But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned.'"