Logical necessity, possibility, and contradiction are not matters of subjective perception or divine will, and genuine contradiction renders at least one concept or aspect of a concept false. An assumption that the Bible never contradicts itself is no more epistemologically valid than any other assumption, which is not at all. Each supposed case is its own and must accordingly be examined on its own. Here, I will examine how 1 Peter 2:18-19 absolutely does seem to contradict Mosaic Law on the righteous treatment of slaves, especially Exodus 21:26-27 and Deuteronomy 23:15-16. I will also address what does and does not logically follow from this. In any case that the New Testament truly contradicted Old Testament ethics, the New would have to be false, since it contradicts its own foundation. Contradicting the inherent truths of logic renders something altogether false regardless of the Bible.
I will present 1 Peter 2:18-19 soon, but I will clarify some key relevant facts. To an extent, without submission, there can be no servitude. Peter telling slaves to submit to their masters under some circumstances is not automatically irrational or unjust. Biblically permitted slavery is very different than some might imagine. Paul also tells slaves to submit to their masters in everything (Colossians 3:22-25), yet he 1) tells masters to not be unjust towards their slaves (Colossians 4:1; Ephesians 6:5-9), which Colossians does not detail the particulars of unlike Mosaic Law, and he 2) separately acknowledges the Torah's laws as righteous and obligatory. He himself attests to this in his own epistles (Romans 3:31, 7:7, 12, 13:8-10, 1 Corinthians 5:13, 9:7-12, 2 Corinthians 13:1, 1 Timothy 1:8-11, 5:17-21, etc.); Luke reports him as saying similar things (Acts 23:1-3, 24:14). Paul, like Jesus, can speak in hyperbole about matters like the submission of slaves but still be entirely consistent in meaning with the exact moral philosophy of the Torah.
Jesus says in Matthew 19:9 that divorce and remarriage to a new partner for reasons other than sexual immorality are immoral, but he also says in Matthew 5:17-19 that he has not come to abolish the Law, which he elsewhere regularly approves of (Matthew 15:1-20, 18:15-17). The Torah's laws attributed to an unchanging God (Exodus 20:22, 21:1, and so on with Malachi 3:6) very obviously allow for divorce for reasons besides sexual immorality (Exodus 21:10-11, Deuteronomy 21:10-14, 24:1-4), as well as indirectly but still by logically necessary extension (Exodus 21:26-27; see also Deuteronomy 15:16-17, related to the great parallels of a slave going free and a marriage ending due to abuse). Matthew 19:9, like some of the other things the Jesus of the gospels says, would at best be an exaggeration to shock people into reflecting on the seriousness of casual or truly baseless divorce. He is otherwise a staunch irrationalist who contradicts his own worldview.
Such is an example of a seeming contradiction between the New and Old Testament. For another example of what might appear like an obvious contradiction between Paul's theology and the Torah's, see 1 Corinthians 7:19. Paul says in this verse that circumcision and uncircumcision do not matter because one should keep the commands of God instead. Of course, circumcision is commanded by God (Leviticus 12:1-3), who also says not to add or subtract from his commands (Deuteronomy 4:2, 12:32). Examining the previous verse clarifies that Paul is speaking figuratively, since he says in 1 Corinthians 7:18 that a circumcised man should not become uncircumcised once he becomes a Christian and vice versa. A circumcised man cannot just "become" uncircumcised again. These examples from the words of Jesus and Paul are clarified by other statements from the same respective figure. In these instances, the clarifications are even in the same book of the Bible.
Peter does not include such a directly illuminating statement affirming the boundaries of submission anywhere in 1 or 2 Peter. Many things are similarly true of the conceptual relationship between the Torah and 1 Peter 2 concerning submission to political authorities rather than masters/mistresses and employers by more immediate extension. I already wrote about this [1] before scheduling each respective article. Again, this post will focus on slaves submitting to masters, which Peter does encourage even when the latter are outright evil. Unless Peter strictly meant this in a supererogatory manner (something is supererogatory if it is morally good but not mandatory), he would be in error:
1 Peter 2:18-19--"Slaves, in reverent fear of God submit yourselves to your masters, not only to those who are good and considerate, but also to those who are harsh. For it is commendable if someone bears up under the pain of unjust suffering because they are conscious of God."
First of all, although I am not assuming that morality exists or that any particular logically possible moral idea is true if morality exists, it cannot be obligatory to submit to evil people. Evil is what should not be done, and to tolerate or submit to evildoers would thus by nature be, at the very least, to think supportively/lightly of immorality or go further by cooperating with them. Besides all of this, the person mistreating someone else is the one in the moral wrong, not the victim for defending himself/herself, fleeing, or verbally challenging emotionalistic, egoistic, and otherwise irrationalistic "authority" figures of every sort.
Mercifully or lovingly submitting in the face of abuse could be morally good but optional, as it could not be required. Contrast what Peter says with what the Torah teaches about just two of the human rights of slaves in particular. It is not merely unstated but logically necessary in light of some other moral concept of Mosaic Law, but it is the explicit, absolute moral doctrine of the Old Testament that all slaves must go free for abuse (at a minimum for physical abuse) and that they all have the right to just walk away without being opposed. The verses below convey such things:
Exodus 21:26-27--"'An owner who hits a male or female slave in the eye and destroys it must let the slave go free to compensate for the eye. And an owner who knocks out the tooth of a male or female slave must let the slave go free to compensate for the tooth.'"
Deuteronomy 23:15-16--"If a slave has taken refuge with you, do not hand them over to their master. Let them live among you wherever they like and in whatever town they choose. Do not oppress them."
It is true that perhaps Peter means verbal abuse should be endured rather than physical mistreatment, but Deuteronomy 23 does not require in itself or by logical extension any limitations on the kind of immoral treatment that might lead a slave to run away. His direct words certainly do not point to this on their own. Even then, verbal as opposed to physical mistreatment does not nullify the fact that evil people cannot deserve loyalty and protection from true justice. Exodus 21:26-27 in particular also has stark marital ramifications, since a slave goes free even if he or she freely promised lifelong service to a master (Deuteronomy 15:16-17); the parallels to marriage and divorce are very distinct and important even though they are not mentioned by the Bible. The Biblical God does not want husbands, wives, slaves, or anyone else to suffer what he defines as injustice or to be pressured to for the sake of supererogatory righteousness. Ironically, 1 Peter 2:18-19 has been treated as in favor of submitting to spousal abuse in conjunction with the very misunderstood 1 Peter 3:1-7.
Some additional things are relevant. Peter's own comparisons to to the sufferings of Christ connects with far more than just the fact that Jesus is supposed to have been sinless and thus not deserving of any punishment whatsoever. The type of suffering Jesus endured is unjust regardless--for one thing, Deuteronomy 25:3 absolutely condemns all degrading punishments, and Roman crucifixion was meant to be degrading--and no one should submit to injustice, but he also did not have to sacrifice himself to begin with. No one does. All such self-sacrifice is optional, and the book of John does teach that Jesus voluntarily gave up his life (John 10:18). The comparison of a slave submitting to injustice to Jesus doing the same hints at Peter potentially recognizing all of this and thus indirectly teaching that such submission is optional. It is likewise important that Peter is also reprimanded by Jesus for his predicted denial of Christ (Matthew 26:33-35), a voice from heaven for his discrimination against Gentiles (Acts 10-11), and Paul for refusing to associate with Gentiles (Galatians 2:11-13) on separate occasions. The man is obviously presented as inconsistent philosophically and flawed in his moral character according to each of these speakers.
On two counts, Peter is by all appearances wrong about slaves and masters: 1) his philosophy of submission as directly presented is false by logical necessity, because submitting to evil people or giving them allegiance cannot be obligatory, and 2) the Torah laws from God that are a moral prerequisite to the entire remainder of the Bible's ethical teachings entail the opposite of what he claims is good. The logically necessary possibility of Christianity being true and its evidentially probable truth, nevertheless, do not depend on whether every single book of the New Testament as chosen by historical figures is entirely true, including in its moral teachings. There also are possible meanings Peter could have intended that do not defy reason or the Torah. At best, it is still merely a handful of verses after 1 Peter 2:18-19 that hint at him really condoning the supererogatory resignation to suffer even when the type of treatment one receives is universally unjust (aka, crucifixion) or when it, though otherwise just, is directed at you without basis.