In Hebrews 6, the writer states:
For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. 7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. 8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
In this passage, the writer makes two points that run contrary to most common teachings among Christians today:
First, a believer can fall away. It is possible for someone who has obeyed the gospel, who has believed in Jesus, who has experienced what it is to be free in Christ and in a covenant relationship with God to turn away and decide he doesn't want to live that life anymore. And the writer makes the point that once I've seen what God has to offer and decline that offer, there is no counter-proposal! God has sent the gift of salvation in Jesus, and if we reject that, we won't get a different way to enjoy fellowship with God.
Second, there are two responses to the gift of God: to reject it, or to bear fruit for God. There is no third option. The gospel invitation demands a response from us, and it demands our willingness to do more than just be "present" and engage in traditional worship once or twice a week. We are called to grow and to produce fruit for God in our lives - and just as a farmer at harvest wouldn't be any happier with dry earth than he would a bed full of thorns, so God responds to the Christian who does not bear fruit in his own life.