In Acts 1:25, what is an apostle?

Question

[Act 1:25 NLT] (25) as an apostle to replace Judas in this ministry, for he has deserted us and gone where he belongs."

What is an apostle?

Answer 1

Christian sources I have encountered regarding what an apostle is generally come from the Roman world. An apostle is viewed as an "emissary" or "envoy":

An apostle (/əˈpɒsəl/), in its most literal sense, is an emissary, from Greek ἀπόστολος (apóstolos), literally "one who is sent off", from the verb ἀποστέλλειν (apostéllein), "to send off".1 The purpose of such sending off is usually to convey a message, and thus "messenger" is a common alternative translation; other common translations include "ambassador" and "envoy".1...

https://en.wikipedia.org/wiki/Apostle

I think this is certainly part of the story. However, I recently discovered that the Jews had apostles before the disciples and it is very instructive as to what a claim to apostleship would have meant in the first century. Apostles were second only to the Sanhedrin, with whom they participated in councils. And they were responsible for publishing circular letters from the Sanhedrin to the synagogues. I think that this feature, of circulating letters, is an important, overlooked aspect of the apostleship:

APOSTLE AND APOSTLESHIP:

By: Kaufmann Kohler Apostle (Greek ἀπόστολοσ, from ἀποστήλλειν, "to send"), a person delegated for a certain purpose; the same as sheliaḦ or sheluaḦ in Hebrew, one invested with representative power. "Apostoloi" was the official name given to the men sent by the rulers of Jerusalem to collect the half-shekel tax for the Temple, the tax itself being called "apostolé." See Theod. Reinach, "Textes Grecs et Romains, etc.," 1895, p. 208, and also Grätz, "Gesch. der Juden," iv. 476, note 21, where Eusebius is quoted as saying: "It is even yet a custom among the Jews to call those who carry about circular letters from their rulers by the name of apostles"; Epiphanius, "Hæreses," i. 128: "The so-called apostoloi are next in rank to the patriarchs, with whom they sit in the Sanhedrin, deciding questions of the Law with them." The emperor Honorius, in his edict of 399, mentions "the archisynagogues, the elders and those whom the Jews call apostoloi, who are sent forth by the patriarch at a certain season of the year to collect silver and gold from the various synagogues" ("Cod. Theodos." xvi. 8, 14, 29. Compare Mommsen, "Corpus Inscr. Lat." ix. 648. See Apostolé).

Grätz, looking for parallels in Talmudical literature, refers to Tosef., Sanh. ii. 6; Bab. 11b, wherein it is stated that the regulation of the calendar or the intercalation of the month, the exclusive privilege of the patriarch, was delegated by him only to representative men such as R. Akiba and R. Meïr, to act for him in various Jewish districts. (Compare also R. H. 25a and elsewhere.) Such delegates in ancient times were also appointed by the communal authority, sheluḦe bet din (delegates of the court of justice), to superintend the produce of the seventh year of release, so that no owner of fruit, fig, and olive trees, or of vineyards, should keep more than was needful for his immediate use—for three meals; the rest was to be brought to the city storehouse for common distribution every Friday (Tosef., Sheb. viii.). The name "delegate of the community" ("sheliaḦ ẓibbur"), given to him who offers the prayers on behalf of the congregation (Ber. v. 5), rests on the principle of representation as it is expressed in the Mekilta on Exodus, xii. 6: "The whole assembly of Israel shall slaughter it." How can a whole congregation do the slaughtering? "Through the delegate who represents it." Accordingly, the elders of the Sanhedrin of Jerusalem addressed the high priest "sheluḦenu usheluaḦ bet din" (our delegate and the delegate of the tribunal) (Yoma 18b). (The "angels of the churches," Rev. ii. 1, 12, 18; iii. 1, 7, 14, are probably also the "delegates of the churches," not angels, as is the general opinion.) Other delegates—"sheluḦim"—are mentioned in the Talmud: "Those sent forth to accomplish philanthropic tasks ["sheluḦe miẓwah"] need fear no disaster on the road" (Pes. 8b). "Those delegated to collect charity ["gabbae ẓedakah"] were always appointed in pairs, and not allowed to separate in order to avoid suspicion" (B. B. 8b). As a rule two prominent men are spoken of as being engaged together in such benevolences as ransoming captives, and similar acts of charity (Abot R. Nathan [A], viii.; Lev. R. v. Compare the "Ḥaburot" of Jerusalem, Tosef., Megillah, iv. 15). Ḥama bar Adda was called "sheliaḦ Zion" (delegate of Zion), as being regularly sent by the authorities of Babylonia to Palestine charged with official matters (Beẓah 25b; Rashi and 'Aruk).

The apostles, known as such from the New Testament, are declared to have derived name and authority from Jesus, who sent them forth as his witnesses (see Luke, vi. 13; Herzog and Hastings, s.v. "Apostles"). But they were also originally delegated by the holy spirit and by the laying on of hands (Acts xiii. 3) to do charity work for the community (see II Cor. viii. 23). "At the feet of the apostles" were laid the contributions of the early Christians to their common treasury, exactly as was done in the year of release in every city (Tos. Shebiit, viii. 1) and in every Essene community (Josephus, "B. J." ii. 8, § 3). "Two and two" the apostles were enjoined to travel (Mark vi. 7; Luke x. 2), exactly as was the rule among the charity-workers (B. B. 8b), and exactly as the Essene delegates are described as traveling, carrying neither money nor change of shoes with them (Josephus, "B. J." ii. 3, § 4; comp. Matt. x. 9, 10; Luke ix. 3, x. 4, xxii. 35; bemaḳḳel we-tarmil, Yeb. 122a). Thus Paul always traveled in the company of either Barnabas or Silas (Acts xi. 30; xii. 25; xv. 25, 30), and was entrusted with the charitable gifts collected for the brethren in Jerusalem (see also I Cor. xvi. 1; II Cor. viii. 4, ix. 5; Rom. xv. 25; Gal. ii. 10); while Barnabas traveled also with Mark (Acts xv. 39, 40). Paul even mentions as "noted apostles who joined the Church of Christ before him his kinsmen and fellow-prisoners, Andronicus and Junia" (Rom. xvi. 7), persons otherwise unknown to us, but who in all likelihood had received no other mission or Apostleship than that of working in the field of philanthropy among the Jewish community of Rome.

The meaning of the term "Apostle," still used in its old sense (Phil. ii. 25) of "Epaphroditus, your apostle [delegate] who ministers to my wants," was, however, already changed in the Christian Church during Paul's time. It became the specific term for the one sent forth "to preach the kingdom of God" either to the Jews, or, as Paul and his disciples, to the heathen world (Mark iii. 14, vi. 7; Luke vi. 13; Rom. xi. 13). "The gospel of the circumcision gave Peter the chief-apostleship of the Jews, the gospel of the uncircumcision gave Paul the apostleship of the Gentiles," according to Gal. ii. 7, 8; and so Paul calls himself an Apostle not of men but of Jesus Christ (Gal. i. 1). So the term "apostles of Christ" became a standing designation (I Thess. ii. 6), and it was confined to those who "saw Christ" (I Cor. ix. 1).

Finally, the number twelve, corresponding with the twelve tribes of Israel, was fixed in the Gospel records (Matt. x. 2; Mark iii. 14; Luke ix. 1; Acts i. 25) in opposition to the apostles of the heathen, who rose in number from one, in the case of Paul, to seventy (Luke x. 1). Even the act of preaching the good tidings concerning the coming Messiah on the part of the wandering delegates of the community (Luke iv. 18; because of which Jesus himself is once called the Apostle [Heb. iii. 1]) was not without precedent in Jewish life, as may be learned from the prayer for good tidings recited every newmoon ("Seder Rab Amram," 33, Warsaw, 1865; compare R. H. 25a and Targ. Yer. to Gen. xlix. 21).

http://jewishencyclopedia.com/articles/1655-apostle-and-apostleship

Here are some examples of circular letters by the hands of Apostoloi.

Here they are the same epistles laid out in a more orderly format, along with the Hebrew.