"He that is not against us is for us." --Jesus, Mark 9:40
"He that is not with me is against me." --Jesus, Matthew 12:30
It is the same Jesus who speaks these words. For abstract analysis these two sayings of Jesus are in irreconcilable contradiction; but in reality they necessarily belong together. Here again we have living experience to prove our case; under the pressure of anti-Christian forces there came together groups of men who confessed the faith unequivocally and who were impelled to seek a clear decision for or against Christ in strict discipline of doctrine and of life. In their struggle these confessing congregations could not but perceive that the greatest of all the dangers which threatened the Church with inner disintegration and disruption lay in the neutrality of large numbers of Christians; they saw in this the true hostility to Christ. The exclusive demand for a clear profession of allegiance to Christ caused the band of confessing Christians to become ever smaller; they saying "he that is not with me is against me" became an actual concrete experience of the Christian Church; and then, precisely through this concentration on the essential, the Church acquired an inward freedom and breadth which preserved her against any timid impulse to draw narrow limits, and there gathered around her men who came from very far away, and men to whom she could not refuse her fellowship and her protection; injured justice, oppressed truth, vilified humanity and violated freedom all sought for her, or rather for her Master, Jesus Christ. So now she had the living experience of that other saying of Jesus: "He that is not against us is for us."
These two sayings necessarily belong together as the two claims of Jesus Christ, the claim to exclusiveness and the claim to totality. The greater the exclusiveness, the greater the freedom. But in isolation the claim to exclusiveness leads to fanaticism and to slavery; and in isolation the claim to totality leads to the secularization and self abandonment of the Church. The more exclusively we acknowledge and confess Christ as our Lord, the more fully the wide range of His dominion will be disclosed to us. --Dietrich Bonhoeffer, Ethics (emphasis mine)