On that Good Friday he transformed the timber of the Cross into an altar on which he sacrificed himself for us.

And as he hung there for those three terrible hours, he did not offer himself alone: he also offered us, the members of his Church which is his Body and for which he gave himself.  For on that Good Friday he looked beyond the crowd in front of him, beyond the towers and minarets of Jerusalem, down the ages to us, and he offered us also with himself to his Father. 

And that two-fold offering of himself and of ourselves, he continues now in Heaven and in the Eucharist: upon his Throne of Glory and in the Most Holy Sacrament of the Altar.  For in the Eucharist there is present, under the outward forms of bread and wine, Our Blessed Lord himself, and it is there that we join with him in offering him to God, and with that perfect Offering we also make our offering of ourselves, what we do and what we are.