In his Oratio 39, an oration on the Festival of Holy Lights, Gregory of Nazianzus writes about various different sorts of baptisms. There is, of course, the baptism in water, and in the Holy Spirit, and so on. The fourth baptism is a baptism of blood and martyrdom, one whose whose purity can't be undone since it is final in a critical way: this one is far more august than all the others, he says, inasmuch as it cannot be defiled by after-stains (39.17). Once you've suffered martyrdom, that's it, no more defilement.
He also names a fifth baptism, which we might call a baptism of penitential tears:
Yes, and I know of a Fifth also, which is that of tears, and is much more laborious, received by him who washes his bed every night and his couch with tears; whose bruises stink through his wickedness; and who goeth mourning and of a sad countenance; who imitates the repentance of Manasseh and the humiliation of the Ninevites upon which God had mercy; who utters the words of the Publican in the Temple, and is justified rather than the stiff-necked Pharisee; who like the Canaanite woman bends down and asks for mercy and crumbs, the food of a dog that is very hungry (39.17).
This baptism is an immersion into tears of compunction, tears of repentance and guilt and sorrow over sins committed. This fifth baptism is a sort of monkish intense penitence over the wrongs done over a life time.
Gregory writes that because he is a man, precisely because he is a fallen human being, he gladly accepts this fifth baptism as a gift from Christ. More importantly, he shares this gift with others because mercy given is mercy received:
I, however, for I confess myself to be a man, — that is to say, an animal shifty and of a changeable nature, — both eagerly receive this Baptism, and worship Him Who has given it me, and impart it to others; and by shewing mercy make provision for mercy. For I know that I too am compassed with infirmity, and that with what measure I mete it shall be measured to me again (39.18).
Here he is referencing Christ's words at Mt 5.7 and elsewhere in the sermon on the mount as well: Blessed are the merciful, for they will be shown mercy. Gregory therefore is eager to accept this mercy from Christ, but not without extending the same mercy to others. Even those who sin gravely are offered a chance to repent, because Gregory knows himself to be a sinner in need of grace, too. As James says, mercy triumphs over judgment (Jas 2.13).
The legitimacy of this baptism was important to Gregory, because there were these extremists at that time who did not allow repentance, who insisted on a strict perfectionism. They followed after one Novatus. This kind of extremism is implausible to Gregory, who considers it pride:
But what sayest thou, O new Pharisee pure in title but not in intention, who dischargest upon us the sentiments of Novatus, though thou sharest the same infirmities? Wilt thou not give any place to weeping? Wilt thou shed no tear? Mayest thou not meet with a Judge like thyself? (39.18)
The worst thing in the world would be to have no mercy upon others during your life time, and yet expect mercy from God; to such persons God will show himself similarly merciless, and then they will understand the folly of their extremism. Gregory contrasts this kind of extreme legalism with the mercy shown by Christ and God in the Bible to numerous characters, whether David or Peter. He asks the powerful question, Art thou not ashamed by the mercy of Jesus, Who took our infirmities and bare our sicknesses; Who came not to call the righteous but sinners to repentance; Who will have mercy rather than sacrifice; who forgiveth sins till seventy times seven (ibid.). Instead he considers that the Novatians are just prideful.
This is the downfall of all legalism: the legalists are in need of mercy as much as anyone else, and perhaps more so because of their strictness; precisely because they don't show it, they won't receive it at all. Here, too, maybe lies a danger for those who want to argue that Christians have no obligation to forgive unconditionally. If repentance or even atonement is a condition of forgiveness, Lord forbid that the Lord should fail to judge you for your unrepentant sins! Who has repented of all his sins among men? No one that I know, that much is sure. May God not hold any of our sins against us; therefore let us not hold any against others, either.
In brief, then, we have to allow room for forgiveness and repentance. We cannot demand of others what we cannot perform ourselves. Let's allow for this baptism of tears, and let's participate in it ourselves, too, while we are at it.