Calvin on the Necessity of Perseverance

"Still, our redemption would be imperfect if he did not lead us ever onward to the final goal of salvation. Accordingly, the moment we turn away even slightly from him, our salvation, which rests firmly in him, gradually vanishes away. As a result, all those who do not repose in him voluntarily deprive themselves of all grace." (Institutes of the Christian Religion, 2.16.1)