I remember a beautiful quote about the soul being like a bird that flies through a house at night. Its entrance out of darkness through the first window is birth, and its exit through the second window back into the darkness is death.
I seem to recall it was by a Christian missionary in the context of converting the Vikings.
Does anyone know what I'm talking about?
Bede's Ecclesiastical History of the English People (written about 731) has a version of this, in Book II, Chapter XIII:
Another of the king's chief men, approving of his wise words and exhortations, added thereafter: “The present life of man upon earth, O king, seems to me, in comparison with that time which is unknown to us, like to the swift flight of a sparrow through the house wherein you sit at supper in winter, with your ealdormen and thegns, while the fire blazes in the midst, and the hall is warmed, but the wintry storms of rain or snow are raging abroad. The sparrow, flying in at one door and immediately out at another, whilst he is within, is safe from the wintry tempest; but after a short space of fair weather, he immediately vanishes out of your sight, passing from winter into winter again. So this life of man appears for a little while, but of what is to follow or what went before we know nothing at all. If, therefore, this new doctrine tells us something more certain, it seems justly to deserve to be followed.” The other elders and king's counsellors, by Divine prompting, spoke to the same effect.
The king in question is St. Edwin of Northumbria; the conversation in the early 600's. Note (as did MJ713, in a comment) that the simile itself is uttered by a pagan councilor of the king, and hence is not, strictly speaking, a "Christian metaphor", even though it occurs in the context of a description of a successful conversion to Christianity.
(A modern song incorporating this simile is Patrick Regan's "That Passed, This is".)
For completeness's sake, here is the original Latin version of the section of Bede provided in kimchi lover's answer:
Cuius suasioni uerbisque prudentibus alius optimatum regis tribuens assensum, continuo subdidit: ‘Talis,’ inquiens, ‘mihi uidetur, rex, uita hominum praesens in terris, ad conparationem eius, quod nobis incertum est, temporis, quale cum te residente ad caenam cum ducibus ac ministris tuis tempore brumali, accenso quidem foco in medio, et calido effecto caenaculo, furentibus autem foris per omnia turbinibus hiemalium pluuiarum uel niuium, adueniens unus passerum domum citissime peruolauerit; qui cum per unum ostium ingrediens, mox per aliud exierit. Ipso quidem tempore, quo intus est, hiemis tempestate non tangitur, sed tamen paruissimo spatio serenitatis ad momentum excurso, mox de hieme in hiemem regrediens, tuis oculis elabitur. Ita haec uita hominum ad modicum apparet; quid autem sequatur, quidue praecesserit, prorsus ignoramus. Unde si haec noua doctrina certius aliquid attulit, merito esse sequenda uidetur.’ His similia et ceteri maiores natu ac regis consiliarii diuinitus admoniti prosequebantur.
And here's the Old English text:
Þæs wordum ōðer cyninges wita and ealdormann geþafunge sealde, and tō ƿǣre sprǣce fēng and þus cwæð: "Þyslic me is gesewen, þū cyning, þis andwearde līf manna on eordon, tō wiðmeteness þǣre tīde þe ūs uncūð is, swālic swā þu æt swǣsendum sitte mid þīnum ealdormannum and þegnum on wintertīde, and sīe fȳr onǣlæd and þīn heall gewyrmed, and hit rīne and snīwe and styrme ūte; cume ān spearwa and hrædlīce þæt hūs þurhflēo, cume þurh ōþre duru in, þurh oþre ūt gewīte. Hwæt, hē on þā tīd þe hē inne bið ne bið hrinen mid ðȳ storme þæs wintres, ac þæt bið ān ēagan bryhtm and þæt lǣsste fæc, ac hē sōna of wintra on þone winter eft cymeð. Swā þonne þis monna līf tō medmiclum fæce ætȳweð; hwæat þǣr foregange oððe hwæt þǣr eftflyge, wē ne cunnun. For þon gif þēos nīwe lār ōwiht cūðlicre and gerisenlicre brenge, þæs weorþe is þat wē þǣre fylgen." Þeossum wordum gelīcum ōðre aldormen and þæs cyninges geþeahteras sprǣcan.
Here's the translation of the Old English (ibid):
Another of the king's counsellors, one of his chief men, assented to his words, and taking up the discussion thus spoke : ' O king, the present life of man on earth, in comparison with the time unknown to us, seems to me, as if you sat at table with your chief men and followers in winter time, and a fire was kindled and your hall warmed, while it rained, snowed, and stormed without; and there came a sparrow and swiftly flew through the house, entering at one door and passing out through the other. Now as long as he is inside, he is not pelted with winter's storm; but that is the twinkling of an eye and a moment of time, and at once he passes back from winter into winter. So then this life of man appears for but a little while ; what goes before, or what comes after, we know not. So, if this new doctrine reports anything more certain or apt, it deserves to be followed.' The other elders and the king's counsellors expressed themselves in similar terms.
The Latin original was written around 731 CE. The Old English translation dates to around 900 CE. The OE doesn't seem much like modern English, does it? But it's actually quite recognizably English if you squint: on wintertīde, and sīe fȳr onǣlæd and þīn heall gewyrmed, and hit rīne and snīwe and styrme ūte; cume ān spearwa and hrædlīce þæt hūs þurhflēo is in wintertime, and the fire laid on and the hall warmed, and it rains and snows and storms out; comes a sparrow and readily that house through flies. Every word is hrædlīce mappable to its modern English equivalent.
(Funny story: when I was studying Old English, we had to translate bits and pieces of Bede, including this passage. The edition we were using was bilingual Latin-Old English. Our professor warned us: "Now, translate from the Old English! Don't cheat by translating from the Latin!" I gave a short laugh, then realized with a shock that not only was he perfectly serious, I was also the only person of the half-dozen or so in the class who actually was entirely incapable of translating the Latin. [I did study Latin later, though.])
