Strictly speaking, followers of religions of the law, namely Jews and Muslims, continue to adhere to the prohibition against eating pork. However, Christians do, in fact, disregard it. There are several reasons for this, among which one can name one essential and several historical ones.

Essentially: the dietary prohibitions (and a whole series of similar restrictions) of the Old Testament were of a temporary nature and were annulled by the New Testament. In the Old Testament, these prohibitions were primarily aimed at protecting the chosen people from detrimental intermingling with pagans. This purpose was served by a whole complex of ideas regarding ritual purity and prohibitions against mixing one thing with another. Among others, the prohibition against eating pork also served this purpose. The relevance of these prohibitions was lost in the New Testament, primarily due to its universality and openness to all nations. Furthermore, and most importantly, the New Testament places the center of gravity in the worship of God in the spiritual realm, not the ritual one (even if the practical life of Christians poorly reflects this).

"Worship in spirit and truth," of which the Lord Jesus Christ speaks in the 4th chapter of the Gospel of John, goes far beyond the bounds of ritual restrictions. Furthermore, the New Testament views impurity exclusively as the result of sin, considering that "there is nothing unclean of itself," except sin. Therefore, the words of Christ, "There is nothing that enters a man from outside which can defile him; but the things which come out of him, those are the things that defile a man... For from within, out of the heart of men, proceed evil thoughts... All these evil things come from within and defile a man" ( 7:15-23) render dietary prohibitions meaningless. A detailed discussion on dietary rituals can also be found in the writings of the Apostle Paul in the 14th chapter of the Epistle to the Romans, where the apostle formulates a view on ritual piety in general and dietary prohibitions in particular as a matter of each person's conscience.

The historical reasons for the lifting of the prohibition on eating pork, related to the aforementioned, stem from the fact that already two decades after the Crucifixion and Resurrection of Christ, a significant proportion, if not the majority, of the Church consisted of Christians from among the Gentiles. For them, the prohibition against eating pork was unfounded, and they did not inherit the ritual life of the Old Testament. When the entry of Gentiles into the Church was just beginning, the Lord showed the Apostle Peter in a vision a multitude of foods forbidden to the Jews (presumably including pork). To Peter, who refused all this, it was said: "What God has cleansed you must not call common" ( 10:15).

After a period of disagreements and disputes over whether Gentile Christians should observe the ritual prescriptions of the Mosaic Law, at the Council of Jerusalem in the year 49, the apostles decreed: "For it seemed good to the Holy Spirit, and to us, to lay upon you no greater burden than these necessary things: that you abstain from things offered to idols, from blood, from things strangled, from sexual immorality, and not to do to others what you would not want done to yourselves. If you keep yourselves from these, you will do well" ( 15:28-29). Thus, dietary restrictions and a whole series of other Jewish ritual prescriptions were formally recognized as non-binding for Christians.

The question is too broad to be given a definitive answer. For each person in specific circumstances, such a question may be answered differently. As far as we can understand the Apostle Paul in  14:1-23, the eating or abstaining from meat is not directly related to one's spiritual life, for "the kingdom of God is not a matter of eating and drinking but of righteousness and peace and joy in the Holy Spirit" ( 14:17). Furthermore, as an example, one may cite the well-known vision of the Apostle Peter,  10:9-20, when God blesses foods to be eaten (including meat) which Peter, as a Jew, considered forbidden. The Gospels do not record that Christ ate meat; however, they omit many other details. In truth, the Gospels do not set out to record such particulars. Nevertheless, since partaking of the Passover lamb, for example, was an integral part of the Jewish observance of Passover, it is reasonable to assume that Christ did so.

The texts you mentioned, "For whoever is not against us is for us" (Mar 9:40) and "Whoever is not with me is against me, and whoever does not gather with me scatters" (Mat 12:30), certainly do not contradict each other. They speak of different, although related, things.

In the Gospel of Matthew, the Lord speaks to a person about his relationship with Him Himself. The main thing He wants to convey to the listeners (and readers) is that it is impossible to remain neutral towards God. God's will for the salvation of the world was that Christ would be revealed to all of us, and our relationship with Him becomes the cardinal choice of human life. In Deuteronomy, there are remarkable words: "I call heaven and earth to witness against you today, that I have set before you life and death, blessing and curse. Therefore choose life..." (Deu 30:19). Thus, our relationship with Jesus Christ is exactly this choice between life and death. It is clear that if a person does not choose life and does not live, then he chooses death—there is no third option, as they say. But this is the relationship between God and man.

When instructing the apostles, however, the Lord Jesus speaks about relationships between people. The primary call of the New Testament here is to always seek peace as much as it is at all possible. The Lord says in the Sermon on the Mount: come to terms quickly with your accuser while you are going with him to court. The apostle Paul writes in one of his epistles: owe no one anything, except to love each other... do not be overcome by evil, but overcome evil with good. By telling the apostles, "whoever is not against us is for us," the Lord is saying that if another person is not directly hostile towards us, then he cannot be an opponent for us either. In fact, Christ's radically new commandment speaks of love even for enemies—all the more reason a disciple of Christ cannot allow hostility and division on their own initiative. Christ repeatedly warned His disciples that their faith would cause hostility from people against them: "the hour is coming when whoever kills you will think he is offering service to God...". But the disciples must not respond in kind.

In practical life, this means that if you are not being nourished by the only Source of life—Christ—then you are not being nourished by anything at all, because there simply is no other source of life. At the same time, if there is an opportunity with someone to build "human," benevolent relationships without compromising communion with Christ, a disciple must not neglect it. And as for the one who is "against you," the Lord calls us to love them too: to pray for them and, if possible, to do good to them as well.