This is where knowing the Greek tense of a verb is very helpful. Verse 14, “We have become partakers of Christ . . . ” (past perfect tense) – “if we hold fast” = if we persevere, it proves we were partakers of Christ in the past and have been partakers – the perfect tense speaks of past action with continuing results.
So, the warning of “take care” is for everybody in the audience, for believers and those that claim to be believers in the church. If a person perseveres, it proves he was justified in the past, if they don’t persevere in the faith, it proves they were not really partakers of Christ in the past, and in that sense “fall away from the living God”.
Warnings to take care and persevere are means that God uses to give us grace to keep on persevering in the faith.