From the Catechism of the Catholic Church §662
The lifting up of Jesus on the cross signifies and announces his lifting up by his Ascension into heaven, and indeed begins it. Jesus Christ, the one priest of the new and eternal Covenant, “entered, not into a sanctuary made by human hands. . . but into heaven itself, now to appear in the presence of God on our behalf”. There Christ permanently exercises his priesthood, for he “always lives to make intercession” for “those who draw near to God through him”. As “high priest of the good things to come” he is the centre and the principal actor of the liturgy that honours the Father in heaven.