Blog 208: 4.14.9 - 4.14.13
Lest we think that we can stir up the faith required to receive the sacraments in a worthy fashion, God grants us his Spirit to accompany his Word and sacrament and to stir our hearts to cling to Christ. "The sacraments properly fulfill their office only when the Spirit, that inward teacher, comes to them, by whose power alone hearts are penetrated and affections moved and our souls opened for the sacraments to enter in" (4.14.9).
If the Spirit doesn't act, then neither the Word nor the sacraments accomplish anything. "The Spirit does this same sort of work in us. For, that the Word may not beat your ears in vain, and that the sacraments may not strike your eyes in vain, the Spirit shows us that in them it is God speaking to us, softening the stubbornness of our heart, and composing it to that obedience which it owes the Word of the Lord." Finally, the Spirit transmits those outward words and sacraments from our ears to our souls" (4.14.10).
This is why the sacraments only have value as instruments that God uses to draw our hearts to him. After all, "God uses means and instruments which he himself sees to be expedient, that all things may serve his glory, since he is Lord and judge of all" (4.14.12). And so, our confidence doesn't rest in the sacrament; rather, it rests in God's glory displayed in and through them as he chooses to use them to confirm and assure our hearts.