This passage, I think, puts the matter rather deftly. An explicit warning begins with verse 12:
12 Βλέπετε, ἀδελφοί, μήποτε ἔσται ἔν τινι ὑμῶν καρδία πονηρὰ ἀπιστίας ἐν τῷ ἀποστῆναι ἀπὸ θεοῦ ζῶντος, 13 ἀλλὰ παρακαλεῖτε ἑαυτοὺς καθ᾽ ἑκάστην ἡμέραν, ἄχρις οὗ τὸ σήμερον καλεῖται, ἵνα μὴ σκληρυνθῇ τις ἐξ ὑμῶν ἀπάτῃ τῆς ἁμαρτίας- 14 μέτοχοι γὰρ τοῦ Χριστοῦ γεγόναμεν, ἐάνπερ τὴν ἀρχὴν τῆς ὑποστάσεως μέχρι τέλους βεβαίαν κατάσχωμεν-...Well, there it is, the danger of unbelief and apostasy, and put in so many words (ἀποστῆναι ἀπὸ θεοῦ ζῶντος-v. 12). The writer is both warning his readers, and enlisting their participation in the "one another" ministry of broadcasting this caution throughout the assembly.
But note well the exact wording of verse 14. The writer does not say, "If we hold fast to our confidence, we shall become partakers." Nor does he say, "We have become partakers of Christ, if we have made a confession of confidence." Nor, "We shall remain partakers of Christ, if we hold fast." Nor even, "We have become partakers of Christ, if we have held fast our confidence."
Rather, he says, μέτοχοι γὰρ τοῦ Χριστοῦ γεγόναμεν, ἐάνπερ τὴν ἀρχὴν τῆς ὑποστάσεως μέχρι τέλους βεβαίαν κατάσχωμεν. The grammar is "We have become X, if we do Y." The first is a first person plural perfect deponent verb, and indicates that "we" have come into a state of being. At some point in the past, this happened, and the effects remain — ἐάνπερ. If what? If we hold fast to the original confidence we placed in Christ, the conviction we came to hold about Him (put as a simple aorist subjunctive).
The wording is interesting, and translation knotty; but my sole focus here is the syntax, and its significance. The writer is saying that present activity reveals past transformation. Perseverance reveals a past work of sovereign grace—and it alone reveals that fact. Not a remembered profession of faith; but enduring possession. Continuance is the test, and proof, of reality.
In this way, the passage is reminiscent of 1 John 5:1—Πᾶς ὁ πιστεύων ὅτι Ἰησοῦς ἐστιν ὁ Χριστὸς, ἐκ τοῦ θεοῦ γεγέννηται. "Everyone who currently, now believes that Jesus is the Son of God has, at a point in the past, been begotten by God."
As regeneration precedes and causes saving faith, so being made a partaker (or partner) of Christ precedes and causes endurance.