15 For not only was baptism not profitable to you, and not also hurtful. Even holy things may be hurtful. In the good, indeed, holy things are to salvation; in the evil, to judgment. For we certainly know, brethren, what we receive, and what we receive is at any rate holy, and no one says that it is not: and what says the apostle? “But he that eats and drinks unworthily, eats and drinks judgment to himself.” He does not say that the thing itself is bad, but that the evil man, by receiving it amiss, receives the good thing which he does receive to judgment.
Was that morsel which the Lord delivered to Judas evil? God forbid. The physician would not give poison; it was health the physician gave; but by unworthily receiving it, he who received it not being at peace, received it unto destruction. So likewise also he who is baptized. I have (baptism), says he, for myself. You have it, I admit. Give good heed to what you have; by that very thing which you have you will be condemned. Wherefore? Because you have what belongs to the dove apart from the dove.
If you have what is the dove's in the dove, you are safe. Suppose yourself a soldier: if you have your general's mark within the lines, you serve in safety; but if you have it out of bounds, not only that mark will not be of advantage to you for service, but you will even be punished as a deserter. Come, then, come, and do not say, I have already, I have enough. Come; the dove is calling you, calling you by her sighing. My brethren, to you I say, call by groaning, not by quarreling; call by praying, by invitation, by fasting; let them by your charity understand that you pity them.
I doubt not, my brethren, that if they see your sorrow they will be astonished, and will come to life again. Come, then, come; be not afraid; be afraid if you do not come; nay, be not afraid, rather bewail yourself. Come, you will rejoice if you will come; you will indeed groan in the tribulations of your pilgrimage, but you will rejoice in hope. Come where the dove is, to whom it was said, “My dove is one, the only one of her mother.” Do you see not the one dove upon the head of Christ, do you not see the tongues throughout the whole world?
It is the same Spirit by the dove and by the tongues: if by the dove the same Spirit, and by the tongues the same Spirit, then was the Holy Spirit given to the whole world, from which Spirit you have cut yourself off, that you might clamor with the raven, not that you might sigh with the dove. Come, then.
Source: Tractates on the Gospel of John (New Advent)