There are warnings to the Jews in Hebrews about falling away. One theological premise is that it can be apostasy representing loss of salvation. That would have to be determined based on the definition of the words used and the context they are in. We have "a great high priest" (Heb 4:14) who David prophesied about saying, "'Today if you hear His voice, do not harden your hearts'" (:7). We are to "draw near" (:16) to him to "find grace to help in time of need" (:16). "The one who has entered His rest has himself also rested from his works" (:10). So we are to be "diligent to enter that rest, so that no one will fall, through following the same example of disobedience" (:11). The example was when Israel failed to enter the promised land. If you don't enter that rest you will fall, and that generation "fell" in the wilderness. They died, so does that mean that if you do not believe, you will not enter in to heaven? These Hebrews were told they had "need again for someone to teach you the elementary principles of the oracles of God" (5:12) because they were "not accustomed to the word of righteousness" (:13). What does it mean that you can "fall" (4:11)? Apparently these Hebrew believers were jeopardizing their position because they were being warned. Their immaturity (:12) was eroding their standing and the solution was to "press on to maturity" (6:1). Without perseverance it is possible to become like them who "have fallen away" (:6).
Answering a particular question without addressing the overall theme it is a part of is almost taking it out of context. But compensating by explaining everything can be overkill. You could begin by citing the introductory statement of God that "in these last days has spoken to us in His Son, whom He appointed heir of all things" (Heb 1:2). It follows that if we don't listen and "neglect" (2:3) it we can "drift away from it" (:1). It is "salvation" (:3) marked by "the beginning of our assurance" (3:14). This is tied to having "once been enlightened and have tasted of the heavenly gift" (6:4). They had shared and had become "partakers of a heavenly calling: (3:1) and "partakers of Christ" (:14). Furthermore God had testified "by various miracles and by gifts of the Holy Spirit: (2:4) and they "have been made partakers of the Holy Spirit" (6:4). The message necessitates that "for this reason we must pay much closer attention" (2:2) to be one "who partakes" (5:13) "therefore leaving the elementary teaching about the Christ" (6:1).