There are a lot of people who call themselves Christians (ie. profess to believe), but have never truly trusted the Lord Jesus Christ for salvation - perhaps they never understood the Gospel, perhaps they know what the Gospel means but were unwilling to repent of their sins, perhaps all they did was say a prayer without knowing the truth of who Jesus truly is or what He did to save us, perhaps they are still trying to work their way to Heaven. All these profess salvation, profess to be saved - but are truly not saved. Ie. they are professors, not possessers - they are not born again, do not have the Holy Spirit indwelling them; therefore it is no surprise that when they get disillusioned about the "Christian life" - which they are perhaps trying to live without having the Holy Spirit inside enabling them to live it - they "reject Christ", and apostasize from the faith - which they never truly possessed to begin with. That is what this passage and Hebrews 6 refers to:

1 John 2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.

Those that left "the faith" - ie. apostasized from the true believers - were never truly of them (ie. true believers). It is not a loss of salvation, but a rejection of their previous profession.