"for whoever is not against us is for us." (Mark 9:40)
"Whoever is not with me is against me, and whoever does not gather with me scatters." (Matthew 12:30)
"What, then, shall we say in response to these things? If God is for us, who can be against us?" (Romans 8:31)
At first glance, these verses seem a bit contradictory. Many just brush them off as contradictory and do not wrestle with the text and nuances of meaning.
The first passage in Mark Has a context of someone driving out evil spirits in Jesus' name. The disciples implied they wanted to stop them from doing so. But Jesus was more concerned about the person being helped than he was that all of their beliefs were perfect. In this context Jesus wants to recognize that when people are helped in his name- even if it is done with wrong motives, it is still a part of the influence of the kingdom. So a cup of cold water given to his follower will be rewarded (9:41). Helping others in his name and helping Christ's people is seen as a good thing. In effect, these people were helping Christ in his work, even though they did not fully understand that.
The Matthew context is different. Jesus had helped someone- driven out a demon- and his critics said he was doing this by the devil. Jesus repled that a house divided against itself cannot stand. He also talked about the unforgivable sin being blasphemy of the Holy Spirit (12:31), and that by their fruits you shall know them. In effect these folks were calling the good work of Christ evil.
The common denominator in both passages is helping people. Those who help others- even with wrong motives because of Jesus are "with" him- doing the work of his kingdom. Those who criticize helping others- or criticize Christ- even if their theology is mainly right- are against his kingdom. There is an old song that asks, "Who is On the Lord's Side?" The next line is "Who will serve the King?" Part of being with Christ is serving him- doing his work. Indeed, it is as Marj Carpenter said, "Mission, Mission, Mission!"
Paul later adds his doxology- "If God is for us, who can be against us?" This basically affirms that no one will be able to stand up against God's power and might in the end. It is simply a reminder that God and one are a majority. It is a reminder that if we seek to be on the Lord's side, in his service, then we will win the day.
Prayer: Lord, give us grace to be on your side, with your mission, in your service.