Communion is a sign and seal of God’s covenant with us. This is often how God works in covenants.
Abraham had faith in God and was already justified, but God gave him the covenant of circumcision, which Paul says is “a seal of the righteousness of the faith which he had while still uncircumcised” (Rom. 4:11).
The faith, the righteousness preceded the sign and seal. It’s this way in family covenants also. At a wedding, the bride and groom take an oath, that’s a covenant, which is signified by rings and sealed in their sexual union.
If communion is only a sign and seal, then why do it? Most importantly, God said to show the covenant this way. Abraham’s faith needed expression, and God wanted to give him a way to pass it on to his children. They were in covenant now, too. James says our faith is seen by what we do.
But second, the sign and seal are means of renewing the covenant. We don’t renew it because it wears out, but because we wear out if we don’t. Husbands and wives renew their love and enjoy their marriage covenant. Calvin said God gave “his church another sacrament, that is, a spiritual banquet, where in Christ attests himself to be the life-giving bread, upon which our souls feed unto true and blessed immortality.”
We are marked and fed here again by Christ’s love.