And you shall take a bunch of hyssop, and dip it in the blood that is in the basin, and strike the lintel and the two side posts with the blood that is in the basin; and none of you shall go out at the door of his house until the morning.
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Ambrose of Milan
AD 397
For he who is baptized is seen to be purified both according to the law and according to the gospel. According to the law, because Moses sprinkled the blood of the lamb with a bunch of hyssop. According to the gospel, because Christ’s garments were white as snow, when in the gospel he showed forth the glory of his resurrection.
They are to bear in mind that those who celebrated the Passover at the time through figures and shadows, when they were commanded to mark their doorpost with the blood of the lamb, marked them with hyssop. This is a mild and humble plant, but it has very strong and penetrating roots. So, “being rooted and grounded in love,” we may be able “to comprehend with all the saints what is the breadth and length and height and depth,” that is, the cross of the Lord. .
Hyssop; Hebrew ezob: which some translate rosemary. (Menochius)
Sprinkle This sprinkling the doors of the Israelites with the blood of the paschal lamb, in order to their being delivered from the sword of the destroying angel, was a lively figure of our redemption by the blood of Christ. (Challoner)
St. Jerome, in Isaias lxvi, says the doors were to be sprinkled in the form of a cross.
Be it so, some will say, in the case of those who ask for baptism; what have you to say about those who are still children and conscious neither of the loss nor of the grace? Are we to baptize them too? Certainly, if any danger presses. For it is better that they should be unconsciously sanctified than that they should depart unsealed and uninitiated. A proof of this is found in the circumcision on the eighth day, which was a sort of typical seal. It was conferred on children before they had use of reason. And so is the anointing of the doorposts, which preserved the firstborn, though applied to things which had no consciousness.
Sprinkle: This sprinkling the doors of the Israelites with the blood of the paschal lamb, in order to their being delivered from the sword of the destroying angel, was a lively figure of our redemption by the blood of Christ.