Regarding enemies within the Church’s own bosom:
From CITY of GOD, XVIII, chap. 51:
Because even when those outside do not rage and there seems to be, and really is, tranquillity, which brings great consolation especially to the weak, even so there are always some, inside indeed there are many, who by their unprincipled behavior torment the feelings of those who live devout lives. For such people cause the name of ‘Christian’ and ‘Catholic’ to be defamed. And the dearer this name is to those who want to live a devout life in Christ, the more they grieve that evildoers within the church make that name less beloved than the hearts of the devout long for it to be.