Human instituted rites are not sacraments.
Lutherans believe that the sacraments are commanded by God, have a visible element (means) and offer God's grace (forgiveness). God's Word is in the sacraments and we know that God's Word is alive and active and gives us and strengthens faith.
The word sacrament is not found in the bible. But neither are other words that we use such as trinity or bible. Lutherans believe that Baptism and the Lords Supper are sacraments since God instituted them and they give the forgiveness to us that was won at the cross by Jesus the Savior.