the geneva catechism 4
310. What is a Sacrament? An outward attestation of the grace of God which, by a visible sign, represents spiritual things to imprint the promises of God more firmly in our hearts, and to make us more sure of them.
311. What? Does a visible and natural sign have this power to assure the conscience? No, not of itself, but in so far as it is ordained of God for this end.
312. Seeing it is the proper office of the Holy Spirit to seal the promises of God in our hearts, how do you attribute this to the Sacraments? There is a great difference between the one and the other. The Spirit of God in very truth is the only One who can touch and move our hearts, enlighten our minds, and assure our consciences; so that all this ought to be judged as His own work, that praise may be ascribed to Him alone. Nevertheless, the Lord Himself makes use of the Sacraments as inferior instruments according as it seems good to Him, without in any way detracting from the power of the Holy Spirit.
313. You think, then, that the efficacy of the Sacraments does not consist in the outward element, but proceeds entirely from the Spirit of God? Yes; for the Lord is pleased to work by these instruments which He has instituted: without detracting from His own power.
314. And what moves God to do that? For the alleviation of our weaknesses. If we were spiritual by nature, like the angels, we could behold God and His graces. But as we are bound up with our bodies, it is needful for us that God should make use of figures to represent to us spiritual and heavenly things, for otherwise we could not comprehend them. At the same time, it is expedient for us to have all our senses exercised in His Holy promises, in order to confirm us in them.
315. Since God has introduced the Sacraments to meet our need, it would be arrogance and presumption to think that we could dispense with them. Certainly: hence he who voluntarily abstains from using them thinks that he has no need of them, condemns Jesus Christ, rejects His grace, and quenches His Holy Spirit.