According to Hebrews 3:12 The ultimate failure of the "evil, unbelieving heart" is that one "falls away from the living God." The Greek word for "falls away" in the Greek is "apostaynai" which could also be translated "defect, revolt, forsake and apostasize". This Greek word is the word from which we get the word "apostasy" in English. This is clearly a case of someone who completely walks away from a relationship with Christ. This happens to be the only place this word is used in Hebrews. As long as any believer lives, they can come to Christ and they can go away from Christ. It is dependent upon a decision of the free will as to where one places and maintains their affection.