John Calvin on Acts 16:31-32

31. Believe in the Lord Jesus. This is but a short, and, to look to, a cold and hungry definition of salvation, and yet it is perfect to believe in Christ. For Christ alone hath all the parts of blessedness and eternal life included in him, which he offereth to us by the gospel; and by faith we receive them, as I have declared, (Acts 15:9.) And here we must note two things; first, that Christ is the mark “Unicum scopum,” the only mark. whereat faith must aim; and, therefore, men’s minds do nothing else but wander when they turn aside from him. Therefore, no marvel if all the divinity of Popery be nothing else but an huge lump “Immane chaos,” immense chaos. and horrible labyrinth; because, neglecting Christ, they flatter themselves in vain and frivolous speculations. Secondly, we must note, that after we have embraced Christ by faith, that alone is sufficient to salvation. But the latter member, which Luke addeth by and by, doth better express the nature of faith, Paul and Silas command the keeper of the prison to believe in the Son of God. Do they precisely stay in this voice [word] only? Yea, it followeth in Luke, in the text, [context,] that they preached the word of the Lord. Therefore, we see how the faith is not a light or dry opinion concerning unknown things, but a plain and distinct knowledge of Christ conceived out of the gospel. Again, if the preaching of the gospel be absent, there shall no faith remain any longer. To conclude, Luke coupleth faith with preaching and doctrine; and after that he hath briefly spoke of faith, he doth, by way of exposition, show the true and lawful way of believing. Therefore, instead of that invention of entangled faith, whereof the Papists babble, let us hold faith unfolded in the word of God, that it may unfold to us the power of Christ.