So Dan P, now that we've established, through your cessation of communication on the subject, that either Peter was fully aware of and preached the mystery Paul referred to (and thus it wasn't a mystery anymore), that God had to some extent united the Gentiles and the Jews in the body of Christ, or that Paul DIDN'T preach some undisclosed mystery (and therefore there's no mystery that should divide us now), shall we go back to the OP and see what we need to discuss about it?
Phil 4:9, which says: "Those things, which ye have both learned, and received, and heard, and seen in me, do: and the God of peace shall be with you." is not mysterious. There is no mystery even hinted at in the verse. Paul is telling the Philipians to follow his teaching and his example, which appears to be along the same lines as the foregoing verses:
Phl 4:1 KJV - Therefore, my brethren dearly beloved and longed for, my joy and crown, so stand fast in the Lord, [my] dearly beloved.
Phl 4:2 KJV - I beseech Euodias, and beseech Syntyche, that they be of the same mind in the Lord.
Phl 4:3 KJV - And I intreat thee also, true yokefellow, help those women which laboured with me in the gospel, with Clement also, and [with] other my fellowlabourers, whose names [are] in the book of life.
Phl 4:4 KJV - Rejoice in the Lord alway: [and] again I say, Rejoice.
Phl 4:5 KJV - Let your moderation be known unto all men. The Lord [is] at hand.
Phl 4:6 KJV - Be careful for nothing; but in every thing by prayer and supplication with thanksgiving let your requests be made known unto God.
Phl 4:7 KJV - And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.
Phl 4:8 KJV - Finally, brethren, whatsoever things are true, whatsoever things [are] honest, whatsoever things [are] just, whatsoever things [are] pure, whatsoever things [are] lovely, whatsoever things [are] of good report; if [there be] any virtue, and if [there be] any praise, think on these things.
In other words, we are supposed to be working together with other believers to show the mind of Christ by our moderation, joy, truthfulness, purity, justness, and loveliness so that Christ will be glorified. To "mystify" such things, especially when Paul didn't, is to hide truth from the world. We don't want to do that.
Thanks for the discussion!
Derf