Why do people disregard the Old Testament…
… is it because they mistake the Scripture which describes the Former Covenant between God and his people, for being the Former Covenant itself - and then taking such verses as Hebrews 8:13* in isolation, they dismiss the entirety of the Tanakh?
Isn't that the most base and rudimentary of Category Errors, and moreover; a twisting of Scripture which St. Paul ascribes to an ignorant and unstable people who do so to their own destruction?
–2Pet 3:16
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* In speaking of a New Covenant, he makes the first one obsolete. And what is becoming obsolete and growing old is ready to vanish away. – Heb 8:13