Jesus said the law is in place until the heavens and earth disappear. How can we learn what the law says without the Torah? First 5 books of the Bible! Most of the prophets are in the Old Testament. The whole Bible is the Word of God. Why would we then reject the majority of the Bible?
Should we study it, or not bother with it? Well I personally study it, but not out of obligation...but out of love for God. I think other people dont want to, and its fine, they dont have to....but Gods word hasnt passed away, its just the mosaic covenant has. Nobody is bound to it anymore. Whats important to God isnt the law but faith. For without faith, its impossible to please God.
Hebrews then lists all the people who exhibited faith in the OT so thats for our learning, not the rituals themselves. Good to know ABOUT them but not actually required to observe those ordinances.
Because we have better covenant made with Jesus blood.
I dont know if the jewish people are actually building a new TEMPLE, but they could be building a new JERUSALEM. In revelation 21 its shown there is no temple inside the new Jerusalem, but theres certainly a bit of changes to the city gates etc. God and the Lamb are the temple.
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