The Onassis Cultural Center is currently hosting a little gem of an exhibition, called Transition to Christianity: Art of Late Antiquity, 3rd - 7th Century AD.
Much of the art and artifacts are on loan from museums in Greece (Athens, Corinth, Thessaloniki, etc.), and some of the objects are on display for the first time in this exhibit. For example, the orans youth pictured above, from 5th/6th c. Maroneia (Thrace) is a new item. (I know what you're thinking: the laity aren't supposed to pray in the orans position -- these post-Vatican II innovations really must be stopped.)
In all seriousness, though, the exhibition offers an almost tactile experience of Christian lived religion in a period of great cultural transition. For example: a wonderfully preserved collection of statuettes from a household shrine (Corinth, 3rd c.), much like the ones that Ambrose's sister, Marcellina, burned incense and prayed before; a hanging lamp in the shape of a fish with a Christogram (chi-rho) on its side; and a collection of apotropaic amulets.
There is also a mensa martyrum, of which I had never seen an example, except in pictures. Most of these "tables/altars of the martyrs" are extant in North Africa, but this one is from Thessaly. It has holes dug out for offerings in memory of the martyrs, whose names are inscribed on the mensa.
Such items, when combined with others dedicated to saints such as Thecla, Menas, and Symeon Stylites, offer a ready reminder of early Christians' devotion to the famous women and men of action in the tradition -- those who are sometimes forgotten when we focus primarily on the great "thinkers."
Some items show subtle transitions to Christianity, such as when the apostles come to be imagined and carved in the postures and gestures of Greek philosophers. But others show sudden, violent shifts, such as a female statue from Rhodes, whose head was defaced and carved into the shape of a cross.
In the end, my favorite items were probably the little signet rings, with molds to impress seals of fish, a dove, a lamb, etc. They reminded me of our earliest textual discussion of what's appropriate in Christian art, from Clement of Alexandria (2nd c.):
"And let our seals be either a dove, or a fish, or a ship scudding before the wind, or a musical lyre, which Polycrates used, or a ships anchor, which Seleucus got engraved as a device; and if there be one fishing, he will remember the apostle, and the children drawn out of the water. For we are not to delineate the faces of idols, we who are prohibited to cleave to them; nor a sword, nor a bow, following as we do, peace; nor drinking-cups, being temperate." (Paedagogus 3.11.59)The exhibit ends fittingly with a solidus coin from Justinian II, which was the first type to put a bust of Christ on the obverse of the coin (and the emperor on the reverse). A transition that was centuries in the making.