John Calvin Commentary Mark 9:40

"For he that is not against us is for us." — Mark 9:40 (ASV)

For he who is not against us is for us. He does not instruct us to give free rein to rash individuals, and to be silent while they interfere in various matters according to their own whims, and disturb the whole order of the Church; for such license, as far as our calling allows, must be restrained.

He only affirms that they act improperly who, at the wrong time, prevent the kingdom of God from being advanced by any means whatsoever. And yet He does not acknowledge as His disciples, or consider as belonging to His flock, those who hold an intermediate position between enemies and friends. Instead, He means that, insofar as they do no harm, they are useful and beneficial: for it is a proverb that reminds us that we should not provoke a quarrel until we are compelled.