Eh, even in the New Testament Jesus instilled in his disciples that the old Jewish law is still acknowledged as the supreme word of God and must continue to be upheld forever. Which is probably why the Old Testament is still a part of pretty much all Christian bibles and is still read and taught.
"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven. For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven." - Matthew 5:17-20
"Do not suppose that I have come to bring peace to the earth. I did not come to bring peace, but a sword. For I have come to turn a man against his father, a daughter against her mother, a daughter-in-law against her mother-in-law--a man's enemies will be the members of his own household. Anyone who loves their father or mother more than me is not worthy of me; anyone who loves their son or daughter more than me is not worthy of me. Whoever does not take up their cross and follow me is not worthy of me. Whoever finds their life will lose it, and whoever loses their life for my sake will find it." Matthew 10:34-39