You are right to point out that the two sayings are only apparently contradictory. They were spoken in different contexts, and while one is about being “with” or “against” the Lord Himself, the other is about being “with” or “for” the Lord’s disciples. Nevertheless, the similarity in the application contexts between the two sayings makes it very clear to us that a significant measure of subjective judgement must be exercised in applying what the Lord is teaching in both verses.

In the first case (Lk 11:23), the Lord had just cast out a demon from a man, and some of the Pharisees are saying that He is casting out demons by Beelzebub (vv. 14–15; cf. Mt 12:24). The Lord refutes their accusations, and then adds: “He that is not with me is against me: and he that gathereth not with me scattereth” (Lk 11:23). In the immediate context, therefore, the Lord is saying that anyone who does not affirm that He was casting out demons by the power of God rather than of Satan is really against Him. There can be no neutrality! But more broadly, what the Lord would be saying is that anyone who does not love Him and confess Him as Lord is really against Him (cf. 1 Cor 16:22; 2 Jn 7).

In the second case (Lk 9:50), the Apostle John had said to the Lord: “Master, we saw one casting out devils in thy name; and we forbade him, because he followeth not with us” (Lk 9:49). It is in answering John, that the Lord says: “Forbid him not: for he that is not against us is for us” (Lk 9:50). It is clear from the context that the Lord is not referring to those who are neutral in so far as His Lordship is concerned. The man who was casting out demons was doing so in His name! John had forbade him not because He was not a disciple of Christ but because he was, for some reason, not following them or ministering to the people with them. Thus, the Lord’s instruction to the disciples was that they should not regard themselves as having the exclusive rights of ministering in Christ’s name. There are those who are not in their band, who are nevertheless for them. The disciples of the Lord should therefore rejoice that there are others who are also serving in the Lord’s name, instead of restricting them.

We are reminded of the event recorded in Numbers 11 where the LORD instructed Moses to gather seventy of the elders of the people so that He might put His Spirit on them. Two of the seventy elders, Eldad and Medad, for some reason, did not gather at the Tabernacle as instructed. But when the Spirit of the LORD descended, the sixty-eight at the Tabernacle began to prophesy, and so did Eldad and Medad! A young man who saw it happening to Eldad and Medad ran to tell Moses. Joshua overheard and was clearly displeased. He said: “My lord Moses, forbid them” (Num 11:28). Moses’ reply was instructive:

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! (Num 11:29).

The Lord’s reply to the Apostle John appears to be in the same spirit as Moses’ reply to Joshua. Joshua and John were both admonished for their inclination to intolerance or narrow exclusivism.

William Hendriksen’s observations concerning the appearance of this tendency in the modern Church is most instructive:

Even today that spirit of narrow exclusivism is at times mistaken for loyalty to one’s church or denomination. We hear people say, “Our denomination is the purest manifestation of the body of Christ on earth.” As long as we are on this sinful earth, a terrain where hypocrisy in high places frequently corrupts not only political but even ecclesiastical life, would it not be better to leave such judgments to God? Let us not be more restrictive than was Moses. Let us not be less broad-minded than was Paul (Phil 1:14–18). Let us follow the teaching of Jesus and, while maintaining what we ourselves regard as purity of doctrine, let us reach out the hand of brotherhood to all those who love the Lord Jesus Christ and build upon the firm foundation of his infallible Word. Doing this, let us pray that we may be instrumental in leading others to the way of salvation, to the glory of God (1 Cor 9:19, 22; 10:31, 33) [Comm. on Luke 9:50].

It may, however, be legitimately asked: Does not the phrase “he that is not against us” admit to a great degree of latitude? What about such as polythesists, pluralists, syncretists and universalists who claim to be friends with us and wish to work with us? Can we build a relationship with them based on the Lord’s words in Luke 9:50? Well, I do not think so, for the Lord’s word in Luke 11:23 would warn us that such as are not with Christ as He is revealed in the Scriptures is really against Him. And in so far as these cannot possibly be true disciples of Christ (Jn 14:6), we ought to have nothing to do with them (2 Cor 6:14–16). But what if they hold to serious doctrinal errors, but may be regarded as basically Christians, such as evangelical Arminians, Dispensationalists and Charismatics? What do we do? Well, perhaps a good way to apply this verse would be to balance between the magnanimity of Hendriksen’s recommendation and the caution of Calvin’s, as reflected in his commentary on the same verse:

[The Lord] does not enjoin us to give a loose rein to rash men, and to be silent while they intermeddle with this and the other matter, according to their own fancy, and disturb the whole order of the Church: for such licentiousness, so far as our calling allows, must be restrained. He only affirms that they act improperly, who unseasonably prevent the kingdom of God from being advanced by any means whatever. And yet he does not acknowledge as his disciples, or reckon as belonging to his flock, those who hold an intermediate place between enemies and friends, but means that, so far as they do no harm, they are useful and profitable: for it is a proverbial saying, which reminds us that we ought not to raise a quarrel till we are constrained (Comm. on Luke 9:50).