The Fourth Sunday of Advent.

In Hebrews it is explained.

Consequently, when Christ came into the world, he said,

Sacrifices and offerings thou hast not desired, but a body hast thou prepared
for me; in burnt offerings and sin offerings thou hast taken no pleasure. Then I
said, Lo, I have come to do thy will, O God, as it is written of me in the roll
of the book.

When he said above, Thou hast neither desired nor taken pleasure in sacrifices and offerings and burnt offerings and sin offerings (these are offered according to the law), then he added, Lo, I have come to do thy will. He abolishes the first in order to establish the second. And by that will we have been sanctified through the offering of the body of Jesus Christ once for all.
Hebrews 10:5-10


“He abolishes the first in order to establish the second.”

The covenants of the Old Testament have been fulfilled - the new covenant is established. The new and everlasting covenant which depends not on human works but on the work of Christ through his Birth, Death on the Cross and Resurrection. A covenant that will never fail.