St Paul says to the Philippians:
Ὥστε, ἀγαπητοί μου, καθὼς πάντοτε ὑπηκούσατε,
μὴ ὡς ἐν τῇ παρουσίᾳ μου μόνον
ἀλλὰ νῦν πολλῷ μᾶλλον ἐν τῇ ἀπουσίᾳ μου,
μετὰ φόβου καὶ τρόμου τὴν ἑαυτῶν σωτηρίαν κατεργάζεσθε·
θεὸς γάρ ἐστιν ὁ ἐνεργῶν ἐν ὑμῖν
καὶ τὸ θέλειν καὶ τὸ ἐνεργεῖν ὑπὲρ τῆς εὐδοκίας.
Wherefore, my beloved, as ye have always obeyed,
not as in my presence only,
but now much more in my absence,
work out your own salvation with fear and trembling,
for it is God which worketh in you
both to will and to do of his good pleasure.
Paul speaks of neither ‘thine own salvation’ nor ‘your own salvations’. Why? The former, ‘thine own salvation’, is naturally avoided, since Paul has up till now been addressing a group of people, and only a major emphasis should compel a switch to a grammatically singular addressee; the latter, on the other hand, ‘your own salvations’, would be easy to say. There must be a reason, it seems, that Paul chooses to use τὴν ἑαυτῶν σωτηρίαν (‘your own salvation’): one salvation. Your salvation and mine are one, for we shall not be saved separately.
It’s entirely true that we have one salvation. That salvation is Christ, is our partaking of him, is our being one with him.
First, then, salvation is one in Christ: we have one Saviour, יהוה alone, who works both willing and doing in us according to his good pleasure. We have no other gods, and we have no other agent of salvation. Nothing in our salvation comes from anywhere else: we ourselves contribute nothing. When our wills and our deeds are conformed to the image of the Christ, it’s God himself who works in our midst and in our spirits. Therefore do we work out our salvation with fear and trembling, because we know that as we work, it’s God himself working in our midst. Fear the awesome presence of יהוה in our work.
Second, our salvation is considered as one, because we are considered as one in our salvation: salvation is the Church as she’s united in marriage to the King of Kings. Here, Paul isn’t contemplating individual salvation: he’s talking about the salvation of the Church, which is incorporated by God alone, not by the works of man. Our salvation cannot be conceived of apart from the Church, because the Church is the substance of our salvation, God the lone actor.
Hear, O Israel: יהוה is our God, יהוה alone, יהוה the Rock of our salvation, יהוה the Bridegroom of the Church. Blessed be the Name of his glorious kingdom for ever and ever.