In explaining the name “Jesus,” Calvin (Institutes 2.16.1) makes this curious statement: “The office of Redeemer was laid upon him that he might be our Savior. Still, our redemption would be imperfect if he did not lead us ever onward to the final goal of salvation.”
That distinction of “redemption” (redemptio) and “salvation” (salutis) is odd in itself. Redemption refers to the payment that Jesus made for our sins, while salvation refers to something that is yet future (progressus ad ultimam). Calvin might be paraphrased as saying, “We are redeemed but not yet saved” or “Our redemption is imperfect (mutila) until we achieve salvation.” Perfect redemption is still to come; it lies ahead in the state that Calvin describes as “salvation.”
Between redemption and salvation, it seems, things can go awry:
“the moment we turn away even slightly from him, our salvation, which rests firmly in him, gradually vanishes.” (Latin: Itaque simulac vel minimum ab eo deflectimus, sensim evanescit salus, quae solida in eo residet.) Salvation resides in Christ. If we have him, we have salvation. But if we turn, salvation recedes, vanishes, slips through our fingers. If we want grace, and eventual salvation, we must “repose” in Him (in eo acquiescunt).
This might be taken in this way: Redemption is imperfect until final salvation is achieved; but it will certainly be achieved for those who are redeemed. The Redeemer is Jesus, Savior, and that means He will perfect our imperfect redemption by bringing us to salvation. If one could possibly turn away from Jesus the Savior, then salvation would vanish. But God has ensured that this won’t happen.
It might mean something riskier: God bought or redeemed some people who won’t make it all the way to salvation. They will glimpse salvation, but it will vanish as they turn from Jesus - because it’s impossible to get to salvation without clinging to the Savior. The exodus provides an analogy: Some of those redeemed from Egypt don’t make it all the way to salus of the promise land.
Calvin says each of these in different places, I think, and the ambiguity has been the source of ongoing reflection and debate among the Reformed, not all of it entirely peaceable.