Impeccable \Im*pec"ca*ble\, a. [L. impeccabilis; pref. im- not +
peccare to err, to sin: cf. F. impeccable.]Of all the four agreements, this is possibly the most difficult. This means not only to be honest, to be truthful... but to speak whole truths, hide nothing. It's like they say in court, "I swear to tell the truth, the whole truth, and nothing but the truth. So help me God."
Not liable to sin; exempt from the possibility of doing
wrong. -- n. One who is impeccable; esp., one of a sect of
Gnostic heretics who asserted their sinlessness.
That is the gist of this agreement.
And breaking this agreement can have incredibly dire consequences.