The first indisputable attestation of a Bishop in the Siwa Oasis
The first indisputable attestation of a Bishop in the Siwa Oasis is given by a recently identified text, a "Historia episcopatus Alexandrie" written towards the end of the 4th century which mentions two Bishops of the oasis ordered by "Theonas d'Alexandrie" between in 282 and 300 AD
Athanasius of Alexandria visited the Ammoniaque in the year 332 with the aim of spreading Christianity in Marmarica.
For now I have not found any information from this visit apart from the fact that Christianity was spreading in the oasis even though in parallel to the maintenance of the old beliefs as demonstrated more than 3 centuries later by Samuel of Kalamoun.
sources: the document was presented by A. Bussi and A. Camplani at the IX Congress of Coptic Studies held in Cairo in September 2008.
Athanasius of Alexandria (296–298 – 2 May 373), also called Athanasius the Great, Athanasius the Confessor or, primarily in the Coptic Orthodox Church, Athanasius the Apostolic, was the 20th bishop of Alexandria (as Athanasius I)