After the Passion of Christ, it is a mortal sin to participate in Jewish rites ~ St. Thomas Aquinas

It is a mortal sin to observe the old rites after the Passion of Christ
The Apostle says (Gal 5:2): “If you be circumcised, Christ shall profit you nothing.” But nothing save mortal sin hinders us from receiving Christ’s fruit. Therefore since Christ’s Passion it is a mortal sin to be circumcised, or to observe the other legal ceremonies.
All ceremonies are professions of faith, in which the interior worship of God consists. Now man can make profession of his inward faith, by deeds as well as by words: and in either profession, if he make a false declaration, he sins mortally. Now, though our faith in Christ is the same as that of the fathers of old; yet, since they came before Christ, whereas we come after Him, the same faith is expressed in different words, by us and by them. For by them was it said: “Behold a virgin shall conceive and bear a son,” where the verbs are in the future tense: whereas we express the same by means of verbs in the past tense, and say that she “conceived …More