Once Saved Always Saved versus Hebrews 3:12

How could "Once Saved Always Saved" be true if Hebrews 3:12 is true? It reads "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God."First, Heb 3:12 is talking about a brother, and most definitely a saved brother in Christ, because verse 1 describes the same as "holy brethren, partakers of the heavenly calling." This cannot be describing an unsaved person no matter how much some want it to do so. It just cannot - unsaved people are not holy and have not partaken of the heavenly calling, not in any sense.Second, Heb 3:12 presents the possibility that such a brother could develop an evil heart of unbelief. This can't mean the former believer is still saved. He has changed to unbelief and John 3:36 says "he that believeth not the Son shall not see life."Third, Heb 3:12 teaches the former believer has departed from God. You can't depart from God unless you were with him to start with, can you? So the clear implication is that a person was with God (saved) and then left God (lost).Ask yourself this question - if God wanted to teach that a Christian can fall from grace, how could he have said it any clearer than he did in Heb 3:12?