For he that is not against us is on our part. He that is not against us, is on our part - Or rather, Whosoever is not against You, is for You. Instead of ἡμων, us, I would read ὑμων, you, on the authority of ADSHV, upwards of forty others, Syriac, Armenian, Persic, Coptic, Ethiopic, Gothic, Slavonic, Vulgate, Itala, Victor, and Opt. This reading is more consistent with the context - He followed not us - well, he is not against You; and he who is not against you, in such a work, may be fairly presumed to be on your side.
There is a parallel case to this mentioned in Numbers 11:26-29, which, for the elucidation of this passage, I will transcribe. "The Spirit rested upon Eldad and Medad, and they prophesied in the camp. And there ran a young man, and told Moses, and said, Eldad and Medad do prophesy in the camp. And Joshua, the servant of Moses, said, My lord Moses, forbid them! And Moses said unto him, Enviest Thou for My sake? Would God, that all the Lord's people were prophets, and that the Lord would put his Spirit upon them." The reader will easily observe that Joshua and John were of the same bigoted spirit; and that Jesus and Moses acted from the spirit of candour and benevolence. See the notes on Numbers 11:25-29 (note).