And on the Sabbath day we went outside the gate [of Philippi] to a riverside, where we were supposing that there would be a place of prayer (proseuchē); and we sat down and began speaking to the women who had assembled. Acts 16:13 (Underline added.)
The Prayer-House at Philippi
One thing that baffles me as I read commentaries on Acts 16:13 is that some people are still unsure what this ‘place of prayer’ (proseuchē) refers to. In my own reading of both primary and secondary texts I have come across the word proseuchē more than a few times, where it is typically used as a synonym for “synagogue”. In particular, it refers to a building where Jews gathered.
In sources from the Second Temple period proseuchē is the most common word used to describe a synagogue building whereas the word synagōgē may indicate a congregation, an assembly, as well as a building or a place of assembly. It is not clear why authors prefer one word to the other, or use the two words alternately . . .”
Luke uses the word proseuchē in Acts 16:13, and again in Acts 16:16. Elsewhere in Acts, Luke uses the Greek word synagōgē (e.g. Acts 17:1).
Proseuchē occurs in Greek literature and in Greek inscriptions where it refers to a building belonging to, or being used by, the Jews of the Diaspora (i.e. Jews who live outside of the land of Israel). Considering the word’s use, I have little doubt that when Paul and his colleagues arrived in Philippi, they went to the river looking for a Jewish meeting place. According to the book of Acts, Paul typically began his missionary work in each new city by going to the local synagogue, and many synagogues (including the prayer-house in Philippi) were built near sources of water, such as rivers, to facilitate ritual washings and baptisms.
The Women at Philippi
The first connection that Paul made in Philippi was with a group of women “who had assembled” (tais synelthousais) in their prayer-house. Synelthousais is a feminine participle of the verb synerchomai, a word which is frequently used in the New Testament with “the idea of a deliberate, purposeful gathering that also implies community.” (This is often true of its use in the book of Acts, but especially true of its use in 1 Corinthians.)
The fact that no men are mentioned in Acts 16:13 is puzzling, but we cannot conclude from this one verse that the Jewish community at Philippi had no adult male members. What we do see in verses 13 and 14, however, is that Paul seems to have had no problem with bringing his message to a group of women, and that “the Lord opened the heart” of one of these women, Lydia, to accept Paul’s message (Acts 16:14).
Lydia, originally from Thyatira, but now settled in Philippi, was probably a Gentile convert to Judaism. She was a wealthy business woman—she dealt in expensive purple fabric—and so Lydia may have been a patron of the Jewish community. Being a patron was an influential role in Roman society and its institutions. Other Jewish women in Philippi may also have played influential roles.
From ancient inscriptions we know that some women in Asia Minor had leadership titles in synagogues during the Roman period. Many titles may have been honorary, but at least some may have denoted genuine leadership functions. Perhaps the Sabbath meetings at Philippi were run and attended mainly by women.
The Jewish women of Philippi weren’t meeting out in the open on a river bank. They had their own building, a proseuchē. It is in this building that they first heard the gospel message from another Jew, the apostle Paul. In Philippi Paul made his first European convert, Lydia. And when she and her whole household were baptised, it was in Lydia’s home that the first Christian church in Europe held its meetings (Acts 16:15, 40).
 Proseuchē literally means “prayer” in Greek, but this word is also used to refer to a Jewish “prayer-house” or synagogue building. Only occasionally does it refer to an outdoor meeting.
 Pieter W. van der Horst, Japheth in the Tents of Shem: Studies on Jewish Hellenism in Antiquity (Peeters: Leuven, 2002), 59.
 For example: 3 Macc. 7:20; various works by Philo of Alexandria including Against Flaccus 7.48; Josephus, Vita 54 (277, 280) see endnote 7; etc.
Jutta Leonhardt states that for Philo of Alexandria “the equivalent of the modern term synagogue was the proseuchē.” Jutta Leonhardt, Jewish Worship in Philo of Alexandria (Tübingen: Mohr Siebeck, 2001), 81, see also 74-95. Louis Feldman concurs: “Indeed, proseuche is the standard term for a place of prayer in Philo (who uses it eighteen times in contrast to synagoge) which refers to the gathered community.” Louis H. Feldman, “Diaspora Synagogues: New Light from Inscriptions and Papyri”, Studies in Hellenistic Judaism (Leiden: Brill, 1996), 580.
 “Greek inscriptions found in Egypt, dating from the third century BCE to the second century CE are the earliest inscriptions mentioning synagogue buildings. All use the term proseuchē to mean a synagogue.” Rachel Hachlili, Ancient Jewish Art and Archaeology in the Diaspora (Leiden: Brill, 1998), 88. (Underline added.)
 Shaye Cohen states that the proseuchē, or ‘prayer house’, “is a product of the Hellenized Diaspora.” Shaye J.D. Cohen, From the Maccabees to the Mishnah (Philadelphia: Westminster Press, 1989), 112, see also 111-115.
While the word is typically used for synagogues outside of Israel, Josephus also refers to a synagogue at Tiberias as a proseuchē. Vita 54 (277, 280). See Lee I. Levine, The Ancient Synagogue: The First Thousand Years (New Haven: Yale University Press, 2005), 52-54, for commentary on Josephus’ description of this synagogue.
 Some “places of prayer” were built by the sea. Josephus records a decree issued by the sea-side city of Halicarnassus which includes this bit: “we have decreed, that as many men and women of the Jews as are willing so to do, may celebrate their Sabbaths, and perform their holy offices, according to Jewish laws; and may make their places of prayer at the sea-side, according to the customs (ethos) of their forefathers . . .” Antiquities of the Jews 14.258 or 14.10.23. (Underline added.) (It is interesting that this decree explicitly mentions women as well as men.)
Note that there is a textual variant of Philippians 16:13. The Textus Receptus has para potamon ou enomizeto proseuchē einai which might be translated as “by a river where there was, according to custom, a place of prayer.”
 Ivoni Richter Reimer, Women in the Acts of the Apostles: A Feminist Liberation Perspective, Linda M. Maloney, trans. (Minneapolis: Fortress Press, 1995), 74. (I’m indebted to Robin Cohn for pointing out this use of synerchomai on her blog post, Women and the Ancient Synagogue of Philippi here.)
 Luke uses the word sebomai (or sebō) eight times in Acts, including Acts 16:14 where it describes Lydia. The verb sebomai literally means “worship”, but it usually occurs in Acts in the context of Gentiles worshipping the God of Israel. The participle form of sebomai is typically translated into English as the idiom “God-fearer” (Acts 13:43, 50; 16:14; 17:4, 17; 18:7).
 For example, a second-century CE inscription from Smyrna mentions a woman named Rufina who was synagogue ruler. It is unclear whether “synagogue ruler” was an actual, or honorary, title in her case; but either way, it indicates she was a woman of influence. The inscription reads: “Rufina, a Jewess synagogue ruler [archisynagōgos], built this tomb for her freed slaves and the slaves raised in her household. No one else has a right to bury anyone here.” (CII 741; IGR IV. 1452)
See Bernadette J. Brooten, Women Leaders in the Ancient Synagogue: Inscriptional Evidence and Background. Brown Judaic Studies, 36. (Chico, CA: Scholars Press, 1982).
 The rule that a synagogue needed to have a quorum of at least 10 adult male members, known as a minyan, may not have been universally observed in the mid-first century CE.
This stream, called Ganga or Gangites, is a short distance north of the ruins of agora in Philippi. However, this stream may not have existed in Paul’s day, or not existed in the same location. The landscape has changed dramatically in more recent times with the draining of marshland around Philippi. (Image source: Ferrel Jenkin’s Travel Blog, used with permission.)
Lydia of Thyatira: The foreigner who became the founding member of the Philippian church
Ministers at Philippi: Women and Men – Philippians 4:2-5
Working Women in the New Testament: Priscilla, Lydia, and Phoebe
Rahab and Lydia: Two Faith-filled Bible Women
The First Century Church and the Ministry of Women
Paul and Women, in a Nutshell
Women and the Ancient Synagogue of Philippi by Robin Cohn here.