It is the virtue of forgiveness, or rather its truth.
By forgiving we mean the fact of erasing a fault, of considering it null and void. This is a power we do not have because the past is irrevocable and all truth is eternal.
To forgive is not to erase, not to forget, it is to accept.
It is to stop hating and this defines mercy, as the virtue that triumphs over rancour, over justified hatred, over the desire for revenge or punishment.
It is important to suppress hatred as much as we can and from there to combat it with joy when this is possible, or with mercy, when joy is impossible or out of place.
Love is a form of joy that does not need to forgive, because as long as we love, mercy is not necessary, love is enough.
Hence his maxim: when you cannot love, at least stop hating.