How is someone "not against" the Lord (Mark 9:38-41)?

Jesus said the following in Mark 9:38-41: "And John answered him, saying, Master, we saw one casting out devils in thy name, and he followeth not us: and we forbad him, because he followeth not us. But Jesus said, Forbid him not: for there is no man which shall do a miracle in my name, that can lightly speak evil of me. For he that is not against us is on our part. For whosoever shall give you a cup of water to drink in my name, because ye belong to Christ, verily I say unto you, he shall not lose his reward." (Mark 9:38-41 KJV)  Does this mean that anyone that claims that "Jesus is Christ" and "I believe in Jesus" is not against Christ? What does it really mean to be "not against" Christ? Does obedience have anything to do with "not being against" Christ?