And I, brethren, if I yet preach circumcision, why do I yet suffer persecution? then is the offense of the cross ceased.
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Augustine of Hippo
AD 430
Since he speaks of a stumbling block here, he is reminding them that the principal reason for the Jews’ taking offense at Christ was that they often saw him ignoring and disdaining those ceremonial observances which they believed themselves to have for their very salvation. What he says here, then, is as much as to say: “It was therefore in vain that the Jews in their indignation crucified Christ when he disdained these commandments. Now they still try to enjoin such things on those for whom he was crucified.”
And I, brethren, if I yet preach. This is a reply to the calumny of the Judaisers, that Paul Judaised among the Jews, and opposed Judaism among the Gentiles. He asks, if this be Song of Solomon , why the Jews should so persecute him, and implies that the real reason is that he publicly opposes them, and condemns circumcision, so as to establish the Gospel.
Then is the offence of the cross ceased. If what they say of me is true, then they are not offended at the Cross which I preach, for they themselves wish to seem Christians, provided only that the Mosaic law may be taken into partnership with the Cross. Nay, the stricter Jews, whose only concern is for Judaism, oppose the preaching of the Cross only because it overturns their law, so much so that they would cease to persecute me if I would combine the law and the Cross. But since, as a matter of fact, they are offended at my preaching, it is obvious that I openly preach the abolition of the law by the Gospel, and the sole sufficienc...
But I, brethren, if I still preach circumcision, why am I still persecuted?
Observe how clearly he exonerates himself from the charge, that in every place he judaized and played the hypocrite in his preaching. Of this he calls them as witnesses; for you know, he says, that my command to abandon the Law was made the pretext for persecuting me. If I still preach circumcision, why am I still persecuted? For this is the only charge which they of the Jewish descent have to bring against me. Had I permitted them to receive the Faith, still retaining the customs of their fathers, neither believers nor unbelievers would have laid snares for me, seeing that none of their own usages were disturbed. What then! Did he not preach circumcision? Did he not circumcise Timothy? Truly he did. How then can he say, I preach it not? Here observe his accuracy; he says not, I do not perform circumcision, but, I preach it not, that is, I do not bid men so to believe. Do not therefore consider it any confir...
Ver. 11. Then has the stumbling block of the cross been done away.
That is, if this which you assert be true, the obstacle, the hindrance, is removed; for not even the Cross was so great an offense to the Jews, as the doctrine that their father's customs ought not to be obeyed. When they brought Stephen before the council, they said not that this man adores the Crucified, but that he speaks against this holy place and the Law. Acts 6:13 And it was of this they accused Jesus, that He broke the Law. Wherefore Paul says, If Circumcision be conceded, the strife you are involved in is appeased; hereafter no enmity to the Cross and our preaching remains. But why do they bring this charge against us, while waiting day after day to murder us? It is because I brought an uncircumcised man into the Temple Acts 21:29 that they fell upon me. Am I then, he says, so senseless, after giving up the point of Circumcision, vainly and idly to expose myself to such injuries, and to place such a stumblin...
Since they were slandering him as one who constantly Judaized and was a hypocrite in his preaching, see how blamelessly he meets the challenge, calling them to admit the truth. “For you know that this is the pretext for persecuting me, that I ask you to avoid the law. But if I preach circumcision, why am I persecuted? For the Jews have nothing to charge me with but this one thing.” … What then? Did he not preach circumcision? Did he not circumcise Timothy? He did indeed circumcise him. How then can he say “I do not preach”? Now learn here his precise meaning. He did not say “I do not perform circumcision” “I do not preach it,” that is, “I do not tell you to believe in this way.”
Of the passion itself should be figuratively set forth in predictions; and the more incredible (that mystery), the more likely to be "a stumbling-stone"