Jim Meritt of Infidels.org claims the Bible contradicts itself here and asks "For or against?" I am including Meritt's notes below each quoted verse to show his reasoning.
|“|| Matthew 12:30 He that is not with me is against me; and he that gathereth not with me scattereth abroad.
(default is against)
Mark 9:40 For he that is not against us is on our part.
(default is for)
Luke 9:50 And Jesus said unto him, Forbid him not: for he that is not against us is for us.
(default is for)
None of the verses say anything about a default. That's simply Meritt's own faulty, so-called reasoning. All three verses simply maintain the same approach, that there are only two sides at work, the world's/Satan's, and God's. Jesus draws the same parallel constantly throughout the Gospels, that there is a kingdom of this world opposing the kingdom of heaven. Therefore, if there are only two sides, it can safely be said that if one is not on the other side, they are on your side, and vice versa - hence, no contradiction.
|“|| Matthew 6:24 No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.
Matthew 8:11 And I say unto you, That many shall come from the east and west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven.
Matthew 13:38 The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one;
Luke 16:8 And the lord commended the unjust steward, because he had done wisely: for the children of this world are in their generation wiser than the children of light.
John 3:19 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
John 8:23 And he said unto them, Ye are from beneath; I am from above: ye are of this world; I am not of this world.
John 12:31 Now is the judgment of this world: now shall the prince of this world be cast out.
John 15:19 If ye were of the world, the world would love his own: but because ye are not of the world, but I have chosen you out of the world, therefore the world hateth you.
John 17:14 I have given them thy word; and the world hath hated them, because they are not of the world, even as I am not of the world.
John 18:36 Jesus answered, My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence.
The constant paradigm shown is that of two kingdoms warring one against the other, Satan's kingdom of this world vs. God's kingdom of Heaven, the prince of this world vs. the prince of Heaven, darkness vs. light. Therefore, if one is not on one side, they are on the other.
- Meritt, Jim (1992). A list of Biblical contradictions. Retrieved from http://www.infidels.org/library/modern/jim_meritt/bible-contradictions.html.