John Calvin Commentary Genesis 21:4

"And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him." — Genesis 21:4 (ASV)

And Abraham circumcised his son. Abraham pursued his steady course of obedience by not sparing his own son. For although it would be painful for him to wound the tender body of the infant, yet, setting aside all human affection, he obeyed the word of God. And Moses records that he did as the Lord had commanded him, because there is nothing of greater importance than to take the pure word of God for our rule and not to be wise above what is lawful.

This submissive spirit is especially required in reference to sacraments, lest people should either invent anything for themselves, or transfer those things which are commanded by the Lord to any use they please. We see, indeed, how inordinately the capricious inclinations of people prevail here, inasmuch as they have dared to devise innumerable sacraments. And to go no further for an example, whereas God has delivered only two sacraments to the Christian Church, the Papists boast that they have seven. As if it were truly in their power to forge promises of salvation, which they might sanction with signs imagined by themselves. But it would be superfluous to relate with how many figments the sacraments have been polluted by them. This is certainly manifest, that there is nothing they are less careful about than observing what the Lord has commanded.