Passage
for he who is not against us is for us;
for he who is not against us is for us;
Mark 9:38 And John did answer him, saying, `Teacher, we saw a certain one in thy name casting out demons, who doth not follow us, and we forbade him, because he doth not follow us.'
Mark 9:39 And Jesus said, `Forbid him not, for there is no one who shall do a mighty work in my name, and shall be able readily to speak evil of me:
Mark 9:40 for he who is not against us is for us;
Mark 9:41 for whoever may give you to drink a cup of water in my name, because ye are Christ's, verily I say to you, he may not lose his reward;
Mark 9:42 and whoever may cause to stumble one of the little ones believing in me, better is it for him if a millstone is hanged about his neck, and he hath been cast into the sea.
The verse centers on "against". It is saying that the verse draws attention to "against" and "Christ", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 39's "And Jesus said Forbid him not for..." into verse 41's "for whoever may give you to drink...", so "against" and "Christ" belong inside that flow. In Mark context, the local focus is Christ, faith, and discipleship.
A plain takeaway is to answer the verse's own emphasis on "against" with trust shaped by these words, not by a vague optimism outside the passage.