19 Neither say, I will not come, because I was baptized without. So, begin to have charity, begin to have fruit, let there be fruit found in you, and the dove will send you within. We find this in Scripture. The ark was made of incorruptible wood. The incorruptible timbers are the saints, the faithful that belong to Christ. For as in the temple the living stones of which it is built are said to be faithful men, so likewise the incorruptible timbers are they who persevere in the faith.
In that same ark, then, the timbers were incorruptible. Now the ark is the Church, it is there the dove baptizes; for the ark was borne on the water, the incorruptible timbers were baptized within. We find that certain timbers were baptized without, such as all the trees that were in the world. Nevertheless the water was the same, not another sort; all had come from heaven, or from abysses of the fountains. It was the same water in which the incorruptible timbers which were in the ark were baptized, and in which the timbers that were without were baptized.
The dove was sent forth, and at first found no rest for its feet; it returned to the ark, for all was full of water, and it preferred to return rather than be rebaptized. But the raven was sent out before the water was dried up. Rebaptized, it desired not to return, and died in those waters. May God avert from us that raven's death. For why did not the raven return, unless because it was taken off by the waters? But on the other hand, the dove not finding rest for its feet, while the water was crying to it on every side, “Come, come, dip yourself here;” just as these heretics cry, “Come, come, here you have it;” the dove, finding no rest for its feet, returned to the ark.
And Noah sent it out a second time, just as the ark sends you out to speak to them; and what did the dove afterwards? Because there were timbers without that were baptized, it brought back to the ark an olive branch. That branch had both leaves and fruit. Let there not be in you words only, nor leaves only; let there be fruit, and you return to the ark, not of yourself, the dove calls you back. Groan ye without, that you may call them back within.
Source: Tractates on the Gospel of John (New Advent)