In Greek this verse continues without a break from verse 19; the Greek aorist passive participle “having been built” is dependent on the main verb “you are” in verse 19 and represents the condition or manner of the readers’ new situation as full members of the Kingdom and family of God. The figure is now that of a building, which carries through verse 22 (verse 20 “build upon”; verse 21 “building”; verse 22 “build together with”). The Greek text says simply “(you) having been built upon”; Good News Translation has You, too are built upon (also Bible en français courant) in anticipation of what is explicitly stated in verse 22, “you are built together with,” and in order to make clear that the experience of Gentile Christians is the same as that of Jewish Christians and not different and separate from it. Jerusalem Bible, Translator’s New Testament have “you are part of a building”; Traduction œcuménique de la Bible “you have been integrated into the building.”

If a time reference is explicit in the aorist participle it is, as Abbott says, to the time when they became Christians.

The figure of a building is expressed by two features: the foundation and the cornerstone. As to the former, the Greek has simply the genitive construction, “the foundation of the apostles and prophets,” which may mean one of four things:

(1) the foundation laid by the apostles and prophets: Bengel, Salmond; Good News Translation, Twentieth Century New Testament, New English Bible, Bible en français courant, Die Bibel im heutigen Deutsch, Biblia Dios Habla Hoy, Nova Tradução na Linguagem de Hoje
(2) the foundation which consists of the apostles and prophets: Abbott, Dodd, Furnish, Robinson, Murray, Beare, Barth; Good News Translation margin, New English Bible margin, Barclay, Traduction œcuménique de la Bible, Moffatt, Goodspeed, New American Bible, Bible de Jérusalem, Jerusalem Bible, Phillips
(3) the foundation upon which the apostles and prophets are built: Alford
(4) the foundation upon which the apostles and prophets have built: Beza

As is to be seen, the majority of commentators and translators prefer number 2, taking “of the apostles and prophets” to be a genitive of apposition (to “the foundation”). Simply to translate “the foundation of the apostles and prophets” (as do Revised Standard Version, Translator’s New Testament, New International Version) is to say in English that the foundation belonged to the apostles and prophets.

Apostles may refer in a restricted sense to the twelve immediate followers and companions of Jesus; more probably it refers to the wider group, which included people like Paul (see 1.1).

Prophets are probably New Testament prophets, people who delivered God’s message in Christian worship, and not Old Testament prophets; but it should be said that the fact that apostles comes before prophets, and that both are governed by one definite article, does not prove that these prophets are not Old Testament prophets. The same phrase occurs in 3.5, where the prophets are, undeniably, Christian prophets.

The bold metaphor which speaks of the believers as being built upon the foundation is something which must often be made a simile in order for the reader to immediately understand that this is a figure of speech, for example, “you, too, have been built, so to speak, upon the foundation.” In some languages the foundation is simply “the stones on which a house is built.”

The phrase laid by the apostles and prophets may be translated as “the apostles and the prophets laid the foundation” or “… made the foundation.” For a treatment of the term apostle see 1.1.

A typical and effective term for prophets is “those who speak on behalf of God” or “those who proclaim God’s word” or “… God’s message.”

If one assumes that the foundation consists of the apostles and prophets, then the figure of speech becomes more complex, because one does not wish to suggest that certain people were simply built literally on top of the apostles and prophets. The foundation, therefore, may be described as “the message (or, truth) which the apostles and prophets proclaimed.”

The cornerstone being Christ Jesus himself: the identification of Christ as the cornerstone makes it more likely that the apostles and prophets are the foundation, but is not conclusive proof that such was the meaning intended by the writer.

It is impossible to determine precisely which stone is meant by the Greek noun translated “cornerstone.” This word occurs also in 1 Peter 2.6, while in Matthew 21.42; Mark 12.10; Luke 20.17; Acts 4.11; 1 Peter 2.7 the phrase “head of the corner” is used. The stone in question could be the large stone used in ancient buildings which extended to the corner of the building, the stones from the wall at right angles to it fitting above and beneath this stone. One such massive stone, 38 feet 9 inches (almost 12 meters) long was discovered in the Temple area in Jerusalem. (Of course it should be noted that in English, at least, the “cornerstone” could refer to the stone at any one of the four corners of an edifice.) Others take the word to mean “keystone,” the stone at the top of an arch. In English this is called the “capstone” or “copestone.” Whatever the precise meaning of the word in Greek, the general meaning is not in doubt: Christ is called the most important stone in the building, the one that provides cohesion and support for the whole structure. So it will be better in most languages to represent the meaning by the phrase “the most important stone.”

In many parts of the world it is not customary to build buildings with foundations or from stones or bricks. As a result the images in this verse pose some difficulties. For these situations foundation can be described as “the solid (or, strong) base put down to build a house on” or “the base people put down before they build a house made of stone.” Cornerstone can then be represented as “the most important stone in the building” or “the stone that gives strength to the building.” When translators use expansions such as these, it becomes much easier to translate with similes, where the comparisons are made clear. One way to do this would be: “You, too, are part of a building. The base they build that building on was put there by the apostles and the prophets, and the stone that gives it strength is Christ Jesus himself.” Another possibility would be: “You, too, are like part of a building for which the apostles and the prophets laid down the base that gives it strength; and Christ Jesus is the important stone that gives strength to the building.”

Quoted with permission from Bratcher, Robert C. and Nida, Eugene A. A Handbook on Paul’s Letter to the Ephesians. (UBS Handbook Series). New York: UBS, 1982. For this and other handbooks for translators see here .