A journey through old Cairo is a journey down the centuries.

THE ritual processions, seen through a curtain of incense fog, evoke the earliest Christian frescoes. The hymns are sung in the vernacular of Pharaonic Egypt, and echo down quiet alleys and along mud-brick walls. Dust is everywhere, thick and heavy, as if just sifted off a tomb. Seconds or centuries could pass here, it doesn't seem to matter.

Threading down the drowsy lanes of ''old Cairo'' is like taking a voyage into another, more mystical realm. Compared with the bedlam outside this quarter, the silence here is almost sepulchral. For a tourist, ''old Cairo'' restores a sense of calm and equilibrium, otherwise lacking in one of the noisiest cities in the world. In Arabic, old Cairo is known as Misr al-Qadimah - ''ancient Egypt'' - and it really is just that. A few minutes' walk in any direction will bring you to the oldest mosque, the oldest synagogue, and the oldest Coptic churches in the country.

More than anything else, old Cairo is Coptic. And there is no better reminder of the Pharaonic past than Egypt's Copts, who claim lineage to the country's ancient inhabitants.

Copts account for an estimated 20 per cent of Egypt's population of 46 million. (For political reasons in this predominantly Muslim country, all statistics regarding the Copts are considered controversial.) They trace their ancestry to the world's first converts to Christianity. The language of their liturgy is that of the Pharaohs, written in Greek letters. In fact, the word Copt, which in Arabic is qubt, is derived from the Greek aigyptos, meaning Egyptian.

Coptic art and ritual are central to an understanding of early Christianity, for no other Christian community has so faithfully preserved its ancient forms. Unlike Hellenistic and Byzantine art, Coptic art has never been bound by traditional styles. Instead, it illustrates how pagan motifs were adapted by the followers of Christ Jesus in Roman Egypt. Coptic churches follow a combination of Roman and Byzantine basilican forms. The interiors of those in old Cairo are veritable museums of ivory-inlaid wood furniture and iconography: primitive, naive in feeling, and slightly African in style.

The quickest, cheapest, and most convenient way to get to the churches in old Cairo is by train, from the Bab el Luq station downtown near Tahrir (''liberation'') Square. The ride lasts 15 minutes, costs the equivalent of 4 cents, and takes you past some of the worst slums in Egypt - from a safe distance. Moreover, you'll avoid the perennial battle with the city's taxi drivers, who don't use meters for foreigners but charge according to your appearance.

The name of the station is Mari Girgis, a very ancient-sounding name which means ''St. George,'' after a monastery and church nearby. You are now in the oldest part of the city. To your right, facing the downtown area you have just left, is a network of gray and off-white cylindrical structures - the fortress ruins of the original Roman city.

Cairo was founded by medieval Arabs upon the ruins of this older Roman town, called Babylon (not to be confused with the Mesopotamian Babylon of earlier antiquity). The original Roman battlements were constructed in AD 98 by the Emperor Trajan, and rebuilt by Arcadius in AD 395, by which time Christianity had triumphed over paganism.

The churches here were built in that murky interregnum between the 4th and 7 th centuries, when Egypt was ruled by Romans, Byzantines, and Arabs in succession. Different guidebooks give different ages for the various churches. No matter. It is enough merely to realize that these buildings are as old as any in the faith.

As you leave Mari Girgis station, beckoning beside the Roman battlements will be the entrance to the Church of the Virgin, called Al-Mu'allaqah (''the suspended''), because its foundations rest atop the remnants of a Roman gate. Here is the earliest known place of Christian worship in Egypt.

Through the vestibule on Sharia Mari Girgis (Mari Girgis Street) is that other, more mystical world. The sounds of the city are left behind, and replaced by birds singing in a narrow courtyard adorned with red flowers and lined with delicately carved wood window screens called mashrabiyyahs.

The best day for a visit is Friday, when the sound of the hymns will quickly draw you away from the courtyard and into the church. Friday, rather than Sunday , interestingly, is the day when Coptic churches are most crowded. This is because in Egypt government offices and many businesses are closed Friday, the Muslim Sabbath. So the Copts take advantage of the holiday to attend church.

From the narrow courtyard of Al-mu'allaqah, a staircase leads to another vestibule, decorated with gnarled marble relief works, which opens into the church proper.

Inside is a theater of carved wood with Roman and Byzantine columns separating the aisles. The altar is cut off from the congregation by a cypress and sycamore screen, inlaid with ivory. Shafts of sunlight, illuminating the incense smoke and the dust, pour through colored glass windows.

Al-Mu'allaqah was destroyed and rebuilt in 840 and has undergone many reconstructions since. On the Friday I visited, the sound of the liturgy was spellbinding. The aisles were packed with worshippers, and the procession truly evoked one of the frescoes in the Coptic Museum next door: priests in white robes overlaid with red brocade sashes, wearing gold miters, and carrying large crosses.

The Coptic Museum in old Cairo has a large collection of early frescoes and marble reliefs. It is open every day but closes early on Fridays.

Next to the Coptic Museum is a lovely garden where the grass is a deep, dark green - a rarity in this desert city - and the paths are lined with 9th-century limestone windows, as well as other archeological treasures. Near the museum ticket window is a narrow lane leading to the Church of St. Sergius (Abu Sarga in Arabic).

A few steps off the alley lead down into the cavernous, subterranean interior of St. Sergius, where the combination of ancient columns, bare stonewalls partitioning the nave, and carved wood create an extremely primitive ambiance. Beneath the altar is a crypt, where, according to tradition, the holy family sojourned during their flight into Egypt to escape the wrath of Herod, who had decreed the death of the child Jesus (Matt. 2:13-21).

Down the street and around the corner from St. Sergius is the Ben Ezra Synagogue, presently marred by scaffolding. It is the oldest Jewish synagogue in Egypt and one of 32 in Cairo - founded in the 12th century by Jerusalem Rabbi Abraham Ben Ezra. The interior is quite oriental, reminiscent of synagogues elsewhere in the former Ottoman Empire. During 19th-century renovations, several hundred medieval manuscripts were uncovered here, written on gazelle skin, and became known as the Geniza Documents.

A few minutes' walk north along Sharia Mari Girgis brings one to the Mosque of Amr Ibn el-As, founded in 641 right after the Arab conquest of Egypt, and the oldest mosque in all of North Africa. Amr Ibn el-As was the Arab conqueror of Egypt, yet nothing remains of the original structure he built. Though a product of many subsequent renovations, the mosque - with its gray concrete facade blending in with the color of the dusty Muqattam hills in the background - appears every bit as old as the churches nearby.

By now one is outside the silent precincts of old Cairo, as the mosque lies just outside the quarter. But, like a visit to a monastery, the few hours spent there renew one's spirit for the confrontation with the outside world.