God has established His covenant in the world, and this covenant is visible. It has visible and external signs, and the covenant people of God gather at particular, identifiable places. They have water placed on them, they hear the Word preached, they sing songs, they pray, they eat the bread and drink the wine.
God has also established His secret elective decree, and He has done this within His own counsels.
We must not confuse this election with the covenant, but neither must we divorce them. They relate to one another, but the only instrument that God has given us for making this relation is faith, not sight. In other words, we cannot see the decree of election in order to understand the covenant. Rather, we believe the promises of the covenant, and this faith (and only this faith) is the instrument that God uses to reveal the outworkings of His elective decree at the Last Day.
Put simply, we are to trust God. We are to believe His signs and seals, we are to believe His promises, we are to rest in hope. Now hope that is seen is no hope at all. Faith is the assurance of things not seen.
This is why we do not trust in the sacraments as objects in themselves—which would be idolatry. Rather, we use them as emblems of faith, and we look through them, believing the promises through them. Do not believe the bread by itself; the bread lies by itself. Do not believe the cup either; the cup lies when you look at it. Believe nothing, believe no one, other than God Himself. He promises, He gives, He bestows Himself. To receive anything else, anything less, is idolatry.
But do not think you can create your own windows, your own sacraments, in order to see and hear His promises. We are to look where He tells us to look, and we are to look the way He tells us to look. In the motions of eating and drinking, sanctified by His Word, we see the ultimate reunion of heaven and earth in the koinonia. And we see this by faith alone.