Jamieson, Fausset and Brown on Genesis 34:13

13. The sons of Jacob answered--The honor of their family consisted in having the sign of the covenant. Circumcision was the external rite by which persons were admitted members of the ancient Church. But that outward rite could not make the Shechemites true Israelites; and yet it does not appear that Jacob's sons required anything more. Nothing is said of their teaching the people to worship the true God, but only of their insisting on their being circumcised; and it is evident that they did not seek to convert Shechem, but only made a show of religion--a cloak to cover their diabolical design. Hypocrisy and deceit, in all cases vicious, are infinitely more so when accompanied with a show of religion; and here the sons of Jacob, under the pretense of conscientious scruples, conceal a scheme of treachery as cruel and diabolical as was, perhaps, ever perpetrated.