The mystery hidden from old in the types and foreshadows of the sin offerings and temple service of the OT was that same mystery the angels longed to look into (1 Pet. 1:12) was how the Father would reconcile mankind to Him; that God had foreordained from the beginning the plan of salvation through the gospel of Christ; that God had always planned to offer His grace to those who would believe of ALL NATIONS, both Jew & Gentile who believed in His Son as the Lamb slain for the last sin offering ever to be offered (Acts 10:10-48).
There is no other sin offering acceptable to the Father than Yeshua, and there is no other way to be reconciled to the Father than thru Christ. The mystery, especially for the Jews was that it was for everyone who will believe.
Col. 1:21-29:
"21 And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled
22 In the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight:
23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;
24 Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:
25 Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God;
26 Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
27 To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
28 Whom we preach, warning every man, and teaching every man in all wisdom; that we may present every man perfect in Christ Jesus:
29 Whereunto I also labour, striving according to his working, which worketh in me mightily." (KJV)