To that end, I am starting this Making Sense of Sunday series. I will be displaying the Second Reading (as found on the USCCB web site, that is, using the NAB Lectionary text, which differs from the NAB) and providing contextual information, glosses, Scriptural cross references, and my own comments. My primary tool will be the Ignatius Study Bibles.
St. Paul's letter to the Ephesians was probably written around A.D. 60, during his first imprisonment in Rome; this is supported by the multiple references to imprisonment found in the letter (3:2, 4:1, 6:20). There is some doubt as to whether the words "in Ephesus" in the greeting of the letter are genuine. If they are genuine, then Paul was writing to Christians in "the leading metropolis of the Roman province of Asia (southwest Turkey)". If they are not genuine, the impression is that this was a "circular letter" to the churches in Asia Minor, in which case Ephesus would have been one of the recipients. Paul did preach in Ephesus (Acts 18-20), so it is logical for him to have written to them.
The church in Ephesus, like many other (notably the church in Thessalonika) was made up both of Jews (Acts 18:19-28; 19:8-10, 17; 20:21) and Greeks (Acts 19:10, 17; 20:21). In some places, this was an occasion of disagreement and disunity, but this was not necessarily a problem in the Ephesian church, although the reading this Sunday does speak of the unifying effect of Christ's crucifixion.
The verses immediately preceding this reading (vv. 11-12) address the Gentiles specifically: "Therefore, remember that at one time you, Gentiles in the flesh, called the uncircumcision by those called the circumcision, which is done in the flesh by human hands, were at that time without [or: separated from] Christ, alienated from the community of Israel and strangers to the covenants of promise, without hope and without God in the world." From these two verses, we enter the reading for this Sunday. The next verse after the reading (v. 19) confirms Gentile Christians as equal-status "fellow citizens" in the "household of God" with the Jewish Christians.
Reading (Eph 2:13-18)
Brothers and sisters:Themes
 In Christ Jesus you who once were far off
have become near by the blood of Christ.
[CCC 2305] For he is our peace, he who made both one
and broke down the dividing wall of enmity,
through his flesh,
 abolishing the law
with its commandments and legal claims,
[Rom 10:12; 1 Cor 12:13;that he might create in himself
Gal 3:28; Col 3:11]one new person in place of the two,
thus establishing peace,
 and might reconcile both with God,
in one body, through the cross,
putting that enmity to death by it.
[Isa 57:19] He came and preached peace to you who were far off
and peace to those who were near,
 for through him
we both have access in one Spirit
to the Father.
- The "far off" (the Gentiles) and the "near" (the Jews) (vv. 13, 17)
- Christ is our "peace" (vv. 14, 15, 17) contrasted with "dividing wall of enmity" (vv. 14, 16)
- Reconciling Gentiles and Jews into "one new person" (vv. 13-17) through his "one body" (v. 16)
- Reconciling through Christ's "blood", "flesh", and "cross" (vv. 13, 14, 16)
- Trinitarian unity (v. 18)
In the two verses before this reading, Paul mentions the conditions (prior to Christ) of the Gentiles and the Jews according to the flesh: "Gentiles in the flesh" and "the circumcision ... done in the flesh by human hands." These fleshly differences disappear through the flesh of Christ. (v. 14) The circumcision of the foreskin which was the sign of the covenant with Abraham is surpassed by the sign of the new covenant in Christ: circumcision of the heart. (cf. Deut 10:16; Jer 9:25-26; Acts 7:51; Rom 2:29) This circumcision is brought about by Christ in the waters of baptism, not by a surgical procedure with human hands. (cf. Phil 3:3; Col 2:11)
The distance between the Jews (those who are "near") and the Gentiles (those who are "far off") is historical as well as liturgical. In the Old Testament, Israel is commanded by God many times to avoid mingling with the other nations, lest they be seduced by false gods and abandon the one true God, the Lord. Thus, Israel tried to keep its distance from the surrounding nations, but failed over and over again. But in the Temple in Jerusalem, there was a "dividing wall" which separated the outer court of the Gentiles from the inner court. A Gentile who passed into this inner area would be punished with death. This wall in the Temple was a liturgical manifestation of Israel's need to keep itself apart from the pagan nations.
Despite this need for separation, there are plenty of prophecies in the Old Testament which speak of a future time when all nations shall worship God, Jew and Gentile together. One of these prophecies is alluded to by Paul. (v. 17; cf. Isa 57:19) By Christ's crucifixion — his blood, his flesh, and his cross — this "dividing wall" is torn down, just as the veil in the Temple was torn as he died. (cf. Matt 27:51) The "access" (v. 18) may also be a reference to the Temple veil, given the other Temple imagery used.
Christ is the "peace" which defeats the "enmity" between the Jews and the Gentiles. This peace comes through reconciliation with God the Father, necessary because of our sins; this reconciliation of peace comes about through Christ's coming in the flesh and the sacrifice of his blood on the cross. The distinctions of Jew and Gentile are lost in Christ: there is "one new person" in the place of the two, a new Israel comprised of all peoples. Jesus, then, is the universal savior and mediator of the new covenant. The one Lord is the Lord of all.
One last point: Paul says Jesus "abolish[ed] the law with its commandments and legal claims." This is not in contradiction to what the Lord himself said in Matthew 5:17: "Do not think that I have come to abolish the law or the prophets. I have come not to abolish but to fulfill." Rather, Christ did fulfill the law and the prophets, instead of doing away with them before they reached their intended conclusion and end. Jesus is the conclusion and end of the law and the prophets. What Paul means, then, is that Jesus (and the early Church, cf. Acts 15) abolished the necessity for Gentiles to become Jews in order to enter into the covenant.