Old and New Covenants

The Old Covenant included a sunset clause, by prophesying a New Covenant. 

To continue keeping the Old Covenant instead of the New once the New Covenant was made, would have been to breach the sunset clause in the Old Covenant itself.

To continue keeping the Old Covenant at the same time as keeping the New Covenant would also have been unlawful, because there can't be two covenants at the same time. 

Therefore the only way to stay compliant with the Old Covenant once the New Covenant came, was to start keeping the New Covenant instead of the Old. By embracing the New instead of the Old, early believers were truly carrying-out the intent of the Old.

And then within that generation, the time came when it became impossible to keep the Old anymore, even if someone wished to. But the New is an eternal covenant.