"But the precepts in what you call your Gospel are so marvelous and great that I don't think that anyone could possibly keep them. For I took the trouble to read them. But this is what surprises us most, that you who claim to be pious and believe yourselves to be different from the others do not segregate yourselves from them, nor do you observe a manner of life different from that of the Gentiles, for you do not keep the feasts or sabbaths, nor do you practice the rite of circumcision. You place your hope in a crucified man, and still expect to receive favors from God when you disregard His commandments."
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This is a quote from Trypho, cited in Justin Martyr's Dialogue with Trypho, the Jew (10.1.2-3). Trypho seems taken aback by the fact that Christians do not separate themselves from unbelievers.