h was gradually lengthened eastwards, being
left comparatively dark, the only light proceeding from the main body of
the church. Simultaneously with or in some cases earlier than these
alterations, a portico known as the narthex was added at the western
end, extending across the whole width of the nave and aisles, for the
use of those, such as catechumens or penitents, who were not privileged
to enter the church itself. The narthex in its turn was set within an
atrium or outer colonnaded court, in the centre of which was a fountain,
used by worshippers for ablutions before entering the consecrated
building.

A minor characteristic of early Christian churches was the richness of
the internal decoration, mosaics that is to say, patterns or pictures
made of many coloured pieces of glass or stone, combined in certain
examples with marble carvings and gilding, adorned the vaulting, the
wall, and even the floor, a kind of mosaic known as the _opus
alexandrinum_ being generally used for the last, the whole producing a
very gorgeous but harmonious effect.

One of the most interesting existing early Christian churches, that
remains very much what it was when first completed, is that of the
Nativity at Bethlehem, built in A.D. 327 by the Empress Helena when on
her quest for the True Cross, with the Convent to which it originally
belonged, that was destroyed by the Turks in 1236 and later restored by
the Crusaders. The Church of the Nativity rises above a natural cave now
converted into a crypt or vaulted subterranean chamber. It is of
cruciform plan, and though its unpretending exterior is of brick, the
interior has four rows of massive stone pillars dividing the nave from
the aisles, which as well as the choir at the eastern end have
semicircular apses.

Contemporary with this humble building, that is closely associated with
all the most sacred memories of the early Church, were the vast
basilican places of worship erected at Rome by Constantine and his
immediate successors, which have unfortunately been either destroyed or
so much modified as to retain little of their distinctive character. The
Cathedral of S. Peter occupies the site of one of them, which had five
aisles, a nave 80 feet wide, a comparatively small apse, and a noble
atrium; the Church of S. Giovanni in Laterano retains but a few details
of its predecessor of the same name, but that of S. Paolo fuori le Mura
or St. Paul without the walls, built by Theodosius i