Laudator Temporis Acti

Revelation 3.20:

Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.

Peter Howell, in his commentary (London, 1980) on book one of Martial (at 1.25.5), writes:

The idea of a benevolent deity waiting outside the house to be let in is both ancient and widespread.

Howell doesn't mention Rev. 3.20, but he does cite some more or less apposite classical parallels. Here they are, with translations.

Aeschylus, Agamemnon 1331-1335 (tr. Herbert Weir Smyth):

'Tis the nature of all human kind to be unsatisfied with prosperity. From stately halls none barreth it with warning voice that uttereth the words "Enter no more."

Plautus, Aulularia 98-100 (tr. Robert Allison):

No one's to enter, while I am away. These are my orders; if Good Fortune comes, please say I am not at home.

profecto in aedis meas me absente neminem
volo intro mitti. atque etiam hoc praedico tibi,
si Bona Fortuna veniat, ne intro miseris.

Martial 1.25.5-6 (tr. anon. from Bohn's Classical Library):

Do you hesitate to admit Fame, who is standing before your door; and does it displease you to receive the reward of your labour?

Ante fores stantem dubitas admittere Famam
    teque piget curae praemia ferre tuae?

Suetonius, Life of Galba 4.3 (tr. J.C. Rolfe):

When he assumed the gown of manhood, he dreamt that Fortune said she was tired of standing before his door, and that unless she were quickly admitted, she would fall a prey to the first comer. When he awoke, opening the door of the hall, he found close by the threshold a bronze statue of Fortune more than a cubit high. This he carried in his arms to Tusculum, where he usually spent the summer, and consecrated it in a room of his house; and from that time on he honoured it with monthly sacrifices and a yearly vigil.

sumpta virili toga, somniavit Fortunam dicentem, stare se ante fores defessam, et nisi ocius reciperetur, cuicumque obvio praedae futuram. utque evigilavit, aperto atrio simulacrum aeneum deae cubitali maius iuxta limen invenit, idque gremio suo Tusculum, ubi aestivare consuerat, avexit et in parte aedium consecrato menstruis deinceps supplicationibus et pervigilio anniversario coluit.

Dio Cassius 64.1.2 (tr. Earnest Cary) repeats the story of Galba:

For it seemed to him in a vision that Fortune told him that she had now remained by him for a long time, yet no one would grant her admission into his house, and that, if she should be barred out much longer, she would take up her abode with somebody else.

The parallel from Aeschylus doesn't seem too close, but the others are interesting, especially those about Galba. I don't have any commentaries on Revelation handy. If I'm not mistaken, the ancients knocked on doors with a kick of the foot, not a rap of the knuckles.

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Update from Kevin via email, who cites Song of Songs 5:2 with a comment:

I was asleep but my heart was awake.
A voice! My beloved was knocking:
"Open to me, my sister, my darling,
My dove, my perfect one!
For my head is drenched with dew,
My locks with the damp of the night."

[New American Standard Bible translation]

Both Jewish and Christian traditions have understood the knocking lover as God. Whether that was the original intention of the author could be a knock-down -- or knock-up -- argument.

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More parallels here.