For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles – (2) if indeed you have heard of the dispensation of the  grace of God which was given to me for you, (3) how that by revelation He made known to me the mystery (as I have briefly written already, (4) by which, when you read, you may understand my knowledge in the mystery of Christ), (5) which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to his apostles and prophets; (6) that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ, through the gospel 

The letter to the Ephesians was written by the Apostle Paul – and written by him while in a Roman prison because of his preaching the gospel to Gentiles.  Paul had founded the church in Ephesus and when he moved on others (Judaizers) arrived in that city; these false teachers also claimed revelation from God and were using Paul’s imprisonment to undermine his authority and his gospel – now Paul was answering them in this letter.  Paul brings up his imprisonment in chapter 2 to dispel slander. He didn’t want the Ephesians to turn from the gospel he preached  or to be discouraged.

The mystery Paul writes of here is utterly new and unexpected; that the Jews and the Gentiles are to be co-heirs to the promises of God. This unity was prophesied in the Old Testament and also addressed by Jesus but has now been revealed with additional clarity – the Gentiles are now equal to the Jews in the sight of God and are to form one body – the condition was that this MUST be accomplished by the gospel. The Jews misinterpreted Old Testament promises of salvation. They thought salvation was the automatic possessions of all Jews. They also believed that only Gentiles who became Jews would be saved – the equal status announced by Paul of Jew and Gentiles was not received well by most Jews

Salvation is by faith through Christ  – Jews were not able to be justified by the Law but only by grace and as this was so, certainly Gentile believers were not under the Law either.