Christians would observe the law, if they did not know why it was instituted
Justin: For since you have read, O Trypho, as you yourself admitted, the doctrines taught by our Saviour, I do not think that I have done foolishly in adding some short utterances of His to the prophetic statements. Wash therefore, and be now clean, and put away iniquity from your souls, as God bids you be washed in this laver, and be circumcised with the true circumcision. For we too would observe the fleshly circumcision, and the Sabbaths, and in short all the feasts, if we did not know for what reason they were enjoined you—namely, on account of your transgressions and the hardness of your hearts. For if we patiently endure all things contrived against us by wicked men and demons, so that even amid cruelties unutterable, death and torments, we pray for mercy to those who inflict such things upon us, and do not wish to give the least retort to any one, even as the new Lawgiver commanded us: how is it, Trypho, that we would not observe those rites which do not harm us—I speak of fleshly circumcision, and Sabbaths, and feasts?
Source: Dialogue with Trypho (New Advent)