system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: the Christian relationship to the Old Testament. If the New Testament came along and changed much of the OT's doctrines, why is the OT still considered just as valid? Why isn't Christianity just based on the NT?
gpt
This is actually a pretty big question in some Christian communities. There is a question as to whether or not the Old Covenant is still "legally" binding to Christians. There are generally three trains of thought;

1) Christ's New Covenant was sealed and put into effect at the Sermon on the Mount, nullifying the Old Covenant.

2) Christ did *not* nullify the Old Covenant at the Sermon on the Mount, and the New Covenant only came into effect either at the Last Supper, the Crucifixion, and/or after he rose from the dead after the three days, at which point the Old Covenant was nullified, and the New Covenant sealed, with the Last Supper being the traditionally held moment of importance. This is also the primary view held throughout most Christian denominations.

3) The Old and New Covenants are both binding, although the New Covenant obviously takes precedence if there's a conflict of doctrine.

This also calls into question what nullifying the Old Covenant means; some believe it means that literally everything in the Old Covenant is no longer important (other than the bits that the Rabbinic Jews refer to as the Seven Laws of Noah, which are basically covered in the 10 Commandments), while others believe that the Old Covenant should still be respected even though it's not important anymore. Furthermore, there are some that believe that the moral codes of the Old Testament are still binding, but the ceremonial codes aren't (which is how you get shellfish-eating homophobes, as well as other cases of people selectively following Old Testament rules).

So in short, to many Christian communities, the Old Testament *isn't* as valid, and is merely taught as a background needed in order to understand the New Testament and the New Covenant (particularly given that Jesus' authority at least partially comes from the prophecy of the Messiah, which is given in the Old Testament).