29. And first of all, by the death of our Redeemer, the New Testament took the place of the Old Law which had been abolished; then the Law of Christ together with its mysteries, enactments, institutions, and sacred rites was ratified for the whole world in the blood of Jesus Christ. For, while our Divine Savior was preaching in a restricted area - He was not sent but to the sheep that were lost of the House of Israel  - the Law and the Gospel were together in force;  but on the gibbet of His death Jesus made void the Law with its decrees  fastened the handwriting of the Old Testament to the Cross,  establishing the New Testament in His blood shed for the whole human race. "To such an extent, then," says St. Leo the Great, speaking of the Cross of our Lord, "was there effected a transfer from the Law to the Gospel, from the Synagogue to the Church, from the many sacrifices to one Victim, that, as Our Lord expired, that mystical veil which shut off the innermost part of the temple and its sacred secret was rent violently from top to bottom." 
30. On the Cross then the Old Law died, soon to be buried and to be a bearer of death,  in order to give way to the New Testament of which Christ had chosen the Apostles as qualified ministers;  and although He had been constituted the Head of the whole human family in the womb of the Blessed Virgin, it is by the power of the Cross that our Savior exercises fully the office itself of Head of His Church. "For it was through His triumph on the Cross," according to the teaching of the Angelic and Common Doctor, "that He won power and dominion over the gentiles"; by that same victory He increased the immense treasure of graces, which, as He reigns in glory in heaven, He lavishes continually on His mortal members; it was by His blood shed on the Cross that God's anger was averted and that all the heavenly gifts, especially the spiritual graces of the New and Eternal Testament, could then flow from the fountains of our Savior for the salvation of men, of the faithful above all; it was on the tree of the Cross, finally, that He entered into possession of His Church, that is, of all the members of His Mystical Body; for they would not have been untied to this Mystical Body through the waters of Baptism except by the salutary virtue of the Cross, by which they had been already brought under the complete sway of Christ.http://www.vatican.va/holy_father/pius_xii/encyclicals/documents/hf_p-xii_enc_29061943_mystici-corporis-christi_en.html
The Old Covenant in the strict sense of the promise
made to Abraham and Moses of course was not really abolished, rather it was fulfilled by the Messiah and the New Covenant and its supreme Divine Sacrifice.
The New Covenant superseded the Old Covenant, the observation of which laws, have no salutary value anymore - and by those knowing Christ, are mortal sin probably even.