Circumcision bound those who observed it to keep the whole ceremonial, judicial, and moral law; baptism binds us to the moral law only, or which is the same thing, to repentance and faith.
Source: The Commentary on the Heidelberg Catechism by Zacharias Ursinus (page 376; emphasis added)
See also page 350:
The sacraments are used lawfully, when the faithful, or such as are converted observe the rites which God has instituted, as signs of grace, and pledges of his will to them. It may be said to consist in these three things: 1. In observing in their purity the rites which God has instituted. Those things which Anti-Christ has added must be removed, and those which have been thrown aside must be restored. This institution of Christ must be retained in its purity. 2. When those observe these rites, for whom God instituted them. None but christians, who by profession of faith, and repents ance are members of the church ought to observe the sacraments. ” If thou believest with all thine heart thou mayest be baptized.” ” And were baptized confessing their sins.” (Acts 8: 37. Math. 3: 6.) 3. When the sacraments are observed with the design for which they were instituted. If any of these conditions are wanting, or if any of the rites are changed, and another design substituted without divine authority; or if the signs be received without faith, it is manifest that the sign and the thing signified do not continue united according to divine appointment. Of those who receive the sacraments it is said: ” Circumcision verily profiteth, if thou keep the law,” &c. (Rom. 2: 25.)
Get that? Sacraments without faith is the same as circumcision without keeping the law.