Leaving Terms Untranslated to Denote Offices
In English there exists a (good, useful) convention (or if it isn't a convention, then a grammatical rule) whereby a word that denotes an office is made distinct from the bare word itself: a Dr. is a teacher, but not every teacher is a Dr., for example.
This applies to the Greek terms for offices in the church also, inasmuch as they have everyday meanings in addition to their being offices in the church.
Our English words bishop, priest and deacon are simply taken from the original Greek words episkopos, presbyteros, and diakonos.1 They were left untranslated in both Latin and English always, because these offices go back as far as the New Testament itself.
This word is comprised of epi (over) and skopos (seer), that is, overseer. Hence other translations include superintendent, guardian. The Latin equivalent is the recognizable word supervisor, comprised again of super (over) and visor (seer).
That is, one who watches over as one responsible—i.e., depending if the context fits, a guardian of said. It's unclear when the tradition began, but bishops began to carry a crosure, which is a symbol of a shepherd, signifying their being bishop in the church.
Acts 20:28 Take heed to yourselves, and to the whole flock, wherein the Holy Ghost has placed you overseers, to shepherd the church of God, which he has purchased with his own blood.
1 Peter 2:10 For you were as sheep gone astray, but now you are turned back to the shepherd and guardian of your souls.
The role of a bishop or overseer is thus to guard, guide, and rule the flock like a shepherd. this role of bishops is why it became traditional for a bishop to carry a crosier (an symbolic or ornamental shepherds crook with which a shepherd saves sheep from straying), as a symbol of his being a shepherd.
One can also make the word something negative by affixing the word other, making it mean, a looker into the affairs of others (i.e. busybody or nosy):
1 Peter 4:15 For let none of you endure suffering as a murderer, or a thief, or an evildoer, or one who looks into the affairs of others (allotriepiskopos).
But this is never the sense this is used outside of the above passage (and indeed, technically, it's a different word).
A bishop is made such by another bishop who lays hands on him, as we shall see is also the case even for deacons, albeit to a different degree.
1 Timothy 4:14 Neglect not the gift that is in you, which was given to you by prophecy with the laying on of the hands of the presbytery.
2 Timothy 1:6 For which reason I remind you to kindle anew the gift of God that is in you by the laying on of my hands.
More on this and its relationship with elder below.
As in English older man and old man are largely synonymous, so in Greek: presbyteros is the comparative form (older man) of presbys (old man). In the Old Testament, in translates zaqin (old man):
Genesis 18:11 Abraham and Sarah were both old (zikenim/presbyteroi), and advanced in years, and the way of women had ceased with Sarah.
However, since the New Testament nowhere explains what it means by elder, or even bishop (i.e. as a new concept, but rather assumes its being known already, merely refining who qualifies for the office), it seems that an analogy is being drawn with the elder concept in the Old Testament: namely, respected quasi-priestly community leader and authority among the people (cf. Mt 15:2; 16:21; 21:23; 26:3, 47; 57, etc.), intimately associated with the dealings of Moses with God, and the establishment of the New Covenant itself (Ex 24:1, 9-10).
Exodus 12:21 And Moses called together all the elders (zikenim/presbyteroi) of Israel and said to them: Take for yourselves a flock for your families, and slay the passover.
With Jesus being the New Moses (Deut 18:15; Acts 3:22 | Ex. 24:8; Mt 26:28), and the Sacrifice of Calvary being commemorated by the Eucharist (Luke 22:11, 17), thus making Him the New Passover (1 Cor 5:7; Jn 19:6; Ex 12:46), it makes sense that those who celebrate the New Passover/the Eucharist are thus called elders.
As should perhaps evident already from the synonimity of oversight and being an elder in a community, but as is also borne out by history, bishop and elder are synonymous insofar as every overseer is also an elder—yet every elder is not necessarily an overseer.
The New Testament was neither intended to be the source of the doctrine that there should be elders and what precise roles and limitations the office has, nor even if it was has it proved to be sufficiently explicit on the precise relationship between the terms. However, writings from a time long before any Christianity had to be 'reverse engineered' from Scripture, instead being conveyed directly from Apostles and disciples thereof to the Christian community (the Scriptures being helps, supports, and divine guidance in the same matters conveyed), are more helpful in this regard.
Namely, Ignatius, bishop of Antioch, writing at the turn of the second century (I.e. at around the same time John wrote Revelation), wrote to the bishops of various churches, addressing one bishop (overseer), but many priests (elders). This is so explicit that it bears no quotation. Suffice it to say he writes at the very end of a long life, being disciples from a very young age. He is not espousing a novel conception of the structure of the church, rather, he addresses single bishop over various cities outside of his own, proving this structure well precedes the first century, and is spread across the world, even to Rome. Indeed, all the earliest writings that deal with successions of bishops (one bishop laying hands on another to succeed in ministry) list one bishop of, for example, Rome, never multiple.
... For your rightly renowned presbytery, worthy of God, is fitted to the bishop as strings to a lyre . (Ignatius, To the Ephesians, cap. 4).
I'n aware of anywhere in either the New Testament or early Christian where bishop and elder are conflated, except when a bishop is called an elder (just like the president is a government official, but not every government official is a president).
The Encyclopedia Britannica asserts (Priesthood):
... Originally the terms presbyteros (“elder”) and episkopos (“overseer”), current in the New Testament and the early church, were probably identical. From the 2nd century on, however, the sacerdotal hierarchy developed along the lines of the Hebrew priesthood, the title episcopus, or bishop, becoming reserved for those who presided over the presbyterate, then called sacerdotes because they shared in the episcopal sacerdotium (“priesthood”), which included the offering of the eucharistic sacrifice of bread and wine. But the conferring of holy orders (ordination of presbyters) and administering the sacrament of confirmation, together with administration of the diocese (jurisdictional area), were confined to the episcopate. In due course the threefold ministry of bishops, priests, and deacons (administrative and liturgical assistants in a parish) became organized on a diocesan basis. This remained the norm in the Western church until the Reformation in the 16th century, when it was repudiated by the continental Reformers (e.g., Martin Luther, John Calvin, and Huldrych Zwingli). In Roman Catholicism, Anglicanism, Swedish Lutheranism, and Eastern Orthodoxy, apostolic succession and jurisdiction have been maintained, especially in the Roman Catholic papacy and in Eastern Orthodox patriarchates. ...
This ignores the fact that Ignatius already speaks of this threefold delineation at the end of the first century, even though he is writing at the culmination of his life, and also assumes the presence of this structure in every church he writes to as already being in place—he is not suggesting or recommending anything, but commenting on the state of things as they already where when we was just about to be martyred in his very old age—having been discipled by none other than Polycarp who literally was discipled by John the Apostle!
As we saw above, the office of bishop is compared in at least some way with Jesus' oversight of the church as the highpriest/shepherd (high priest, not the only priest!), while the office of elder loosely corresponds to Old Testament priesthood in that it is associated with a central sacrifice, namely, the passover. This perhaps explains the existence of the bishop's mitre, which resembles what is spoken of in the Old Testament concerning the attire of the highpriest, namely the turban or headdress, as well as the bishop's crosier (shepherds crook). Every bishop is in a sense a local representation or instantiation of Jesus, as it were, and every priest therefore a representation fo His inner circle of Apostles. Ignatius puts it this way:
For, since you are subject to the bishop as to Jesus Christ, you appear to me to live not after the manner of men, but according to Jesus Christ, who died for us, in order, by believing in His death, you may escape from death. It is therefore necessary that, as you indeed do, so without the bishop you should do nothing, but should also be subject to the presbytery, as to the apostle of Jesus Christ, who is our hope, in whom, if we live, we shall at last be found. It is fitting also that the deacons, as being the ministers of the sacraments of Jesus Christ, should in every respect be pleasing to all. For they are not ministers of meat and drink, but servants of the Church of God. They are bound, therefore, to avoid all grounds of accusation against them, as they would do fire. (Ignatius, To the Trallians, 2).
Not that all of these developments such as vestments and crosisers date back to the Apostles, of course, only that they naturally fruit forth from the fundamental identify of the office itself., which is more kernel-like, and has always been present, East and West, North and South, since the beginning of the church.
Diakonos does not mean servant in the sense of one enslaved to another, but one who works in the service of others, such as the servants of a King (Esther 2:2). Another translation might be minister or attendant.
Acts 6:2-3 And so the Twelve, calling together the multitude of the disciples said: It is not fitting for us to leave the word of God to serve tables. Therefore, brethren, choose from among you seven approved men, full of the Spirit, and of wisdom, whom we may appoint to this purpose. But as for us, we will persevere in prayer and in the service of the word. And the word pleased all the multitude, and they chose Stephen, a man full of faith and of the Holy Spirit, and Philip, and Prochros, and Nicanor, and Timon, and Parmenas, and Nicholas, an Antiochean convert, whom they stood before the Apostles; and praying, they laid hands on them.
This was the first creation of deacons, or the office of servant in the church. There are only a few Apostles, yet many Christians must be attended to as to the distribution of sacraments and other things, and so the Apostles' hands are laid on worthy men chosen from among the Christian disciples, making them men with a distict set-apart role in the church.
In the second century, in writing his Defense to the Romans defending the innocuity of Christianity (contrary to accusations it involved itself in cannabalism, because of the Eucharist—among other accusations), Justin Martyr wrote what what deacons did in the early church during the celebration of the Eucharist (today called Mass or Divine Liturgy or Qurbana/the Sacrifice):
. . . those who are called by us deacons give to each of those present to partake of the bread and wine mixed with water over which the thanksgiving was pronounced, and to those who are absent they carry away a portion (First Apology, 65).
This seems to be identical to what is referred to, in fact, in the quote from Acts above—in any case, per the records themselves, this was what deacons did in the early church.
Suffice to say this too was an office, not just a bare noun or description. They were worthy men selected from among the people to this purpose. Making them holy, giving them a distinct role in the church.
Contrary to the former office of elder (bishop being a kind of elder merely, not a separate office as such), the office of deacon has no precedent in the Old Testament.
Since offices are clearly outlined in the New Testament (although the distinction between elder and bishop is not as clear as the distinction between bishop and deacon), we ought to leave untranslated terms which denote the office (or at least in some other way denote it, by capitalizing the word), when they do, as it has been done quite literally for 2000 years—to admit the church has been wrong from the beginning is to admit the church has been wrong, period—and there is no other church to be found, nor did Christ found another.
1 When we remove what in Greek is merely a grammatical fluff (i.e. not part of the word, the final os), we get episkop, presbyter, and diakon. Which in Latin form becomes episcop, presbyter, and diacon (Greek k in Latin = c). From here it's easy to see how we end up with the English bishop, priest (through Germanic priester), and deacon. It's thus etymologically illiterate to assert that translating presbyter by priest in English is some kind of anachronism or importing of later conceptions of priesthood, since the Old Testament priests (and pagan priests for that matter) are so called in English precisely only by analogy to the Christian elder, and his role, not the other way around!