Every sacrament, by definition, contains and manifests a promise of salvation. That is what a sacrament is. The sacrament of baptism contains a promise of salvation at its inception, and the Supper contains a promise of salvation related to perseverance. What God began God will complete. These promises are apprehended with the heart, whenever someone receives them in faith.
Faith alone, sola fide, is not only alone with regard to works. We are justified by faith alone, but never by a faith that is alone. This is the issue of faith alone related to works, which is an important issue. But there is another sense in which faith is never alone. Faith is never self-originating. Faith is something that is always responsive to something outside of itself that God offers to us. And God offers salvation in two principal places—the first is in the preaching of the Word, and the second is in the presentation of the sacraments. And so baptism and the Supper cannot justify you any more than hearing a sermon can. If you hear the sermon in faith, then you meet Christ. If you partake of the bread and wine in evangelical faith, then you meet Christ.
Because a sacrament must contain a promise of salvation, it follows from this that only God can institute a sacrament . . . because He is the only one who can promise salvation. This is why there are only two sacraments—if we could promise salvation by ritual means, we could generate as many sacraments as we wanted. But we have no authority to issue promises of salvation. We must be content with the authority we were given, which is the authority to believe promises of salvation.