There are many Christians who regard themselves as almost exclusively New Testament believers, arguing that the Old Testament was for the Jews under the dispensation of law and thus not applicable to Christians today.
Nothing could be further from the truth. While the old animal sacrifices, temple rituals, and Levitical priesthoods have indeed been superseded by Christ’s "one sacrifice for sins for ever" (Hebrews 10:12), there are many "exceeding great and precious promises" (2 Peter 1:4) of the Old Testament that can be properly and joyfully appropriated by Christians. "All scripture is given by inspiration of God, and is profitable," wrote Paul (2 Timothy 3:16), speaking particularly of the Old Testament Scriptures.
In the context of our verse for the day, Paul is stressing that his own new revelations, given in connection with the Christian gospel, actually involved bringing Jew and Gentile together as one body in Christ. The "dispensation of the grace of God . . . by revelation he made known unto me," he wrote, but in previous ages, it had not been "made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit" (Ephesians 3:2-5).
And what was it that had not been made known? The hidden mystery was simply "that the Gentiles should be fellowheirs" with the Jews, and therefore "partakers of his promise in Christ by the gospel" (Ephesians 3:6).
Thus Gentile believers can now share in all the gracious promises of God in the Old Testament (e.g., Psalm 23; Isaiah 26:3; etc.), except those directly dealing with the future of Israel as a nation, "that the blessing of Abraham might come on the Gentiles through Jesus Christ" (Galatians 3:14). HMM http://www.icr.org/article/6135/