The following is a section from The Decades III.6 in which Bullinger cites Lactantius from Lib. Instit. Iv. Chap 17:
The meaning of circumcision was, that we should make bare our breasts; to wit, that we should live with a simple and plain dealing heart; because that part of the body, which is circumcised, is partly like to a heart, and an object of shame: and the cause why God commanded to make it bare was, that by that sign he might admonish us not to have a covered heart, that is, that we should not cover the secrets of our conscience any crime whereof we ought to be ashamed. And this is the circumcision of the heart, whereof the prophet speak, which Hod hath translated from the mortal flesh to the immortal soul. For the Lord being wholly set and fully minded, according to his eternal goodness, to have a care for our life and safeguard, did set repentance before our eyes for us to follow, as a way to bring us thereunto: so that, if we make bare our hearts, that is, by confession of our sins we satisfy the Lord, we should obtain pardon, which is denied to the proud and those that conceal their faults by God, who holdeth not the face as man doth, but searcheth the secrets of the breast.