The mystery was that the Gentiles could receive salvation through the FALL of Israel instead of the PROPHESIED RISING of Israel.
Rom 11:11-14 KJV I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy. (12) Now if the fall of them be the riches of the world, and the diminishing of them the riches of the Gentiles; how much more their fulness? (13) For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office: (14) If by any means I may provoke to emulation them which are my flesh, and might save some of them.
Rom 11:25 KJV For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
Compare this with prophesy.
Isa 60:1-5 KJV Arise, shine; for thy light is come, and the glory of the LORD is risen upon thee. (2) For, behold, the darkness shall cover the earth, and gross darkness the people: but the LORD shall arise upon thee, and his glory shall be seen upon thee. (3) And the Gentiles shall come to thy light, and kings to the brightness of thy rising. (4) Lift up thine eyes round about, and see: all they gather themselves together, they come to thee: thy sons shall come from far, and thy daughters shall be nursed at thy side. (5) Then thou shalt see, and flow together, and thine heart shall fear, and be enlarged; because the abundance of the sea shall be converted unto thee, the forces of the Gentiles shall come unto thee.
and more:
Isa 60:18-22 KJV Violence shall no more be heard in thy land, wasting nor destruction within thy borders; but thou shalt call thy walls Salvation, and thy gates Praise. (19) The sun shall be no more thy light by day; neither for brightness shall the moon give light unto thee: but the LORD shall be unto thee an everlasting light, and thy God thy glory. (20) Thy sun shall no more go down; neither shall thy moon withdraw itself: for the LORD shall be thine everlasting light, and the days of thy mourning shall be ended. (21) Thy people also shall be all righteous: they shall inherit the land for ever, the branch of my planting, the work of my hands, that I may be glorified. (22) A little one shall become a thousand, and a small one a strong nation: I the LORD will hasten it in his time.
Compare this with Rev 21.
Rev 21:22-27 KJV And I saw no temple therein: for the Lord God Almighty and the Lamb are the temple of it. (23) And the city had no need of the sun, neither of the moon, to shine in it: for the glory of God did lighten it, and the Lamb is the light thereof. (24) And the nations of them which are saved shall walk in the light of it: and the kings of the earth do bring their glory and honour into it. (25) And the gates of it shall not be shut at all by day: for there shall be no night there. (26) And they shall bring the glory and honour of the nations into it. (27) And there shall in no wise enter into it any thing that defileth, neither whatsoever worketh abomination, or maketh a lie: but they which are written in the Lamb's book of life.