Some background context:
John the Revelator, writing and speaking after the time of Christ, while an unbroken line of Apostles were still alive, prophesying about a time yet to come in the future (from his perspective)
Summary:
John sees an angel from heaven, carrying the 'everlasting gospel, to the earth so that it can be preached to every person thorough the world.
Firstly, as I have pointed out, the everlasting gospel can refer to nothing but the gospel as taught by Christ and the Apostles; this is the only everlasting covenant God has made with men on earth.Secondly, the angel is bringing the gospel from heaven: if the gospel were already (or still) on the earth at the time, why would it need to be brought? The angel would only need to come to encourage men to spread the word. Yet John clearly says, prophetically and by inspiration from God, that an angel would need to come to the earth and bring the gospel back with them.
Thirdly, the church that this gospel was restored to would take it to every part of the world; therefore it must be a proselyting and missionary church.
Conclusions:
For there to be any need of the gospel to be brought back to the earth, the gospel cannot have been present on the earth in any form recognised or accepted by God. If it were, He would not need to send an angel, He could simply speak to the prophet, and even if He did choose to send an angel, they would not be bringing to gospel to the earth, merely visiting the saints of God. therefore, sometime after the time of Jesus Christ (and after John the Revelator) there must be an apostasy, absolute and complete, such that the gospel in its fulness, and the authority that goes hand in hand, no longer reside anywhere on the earth. Therefore, any church claiming to have (or that it will have) continuous, unbroken lineage of leaders from Christ’s mortal ministry until His return in glory, is mistaken (at the very least and to put it lightly). Any church that is true must be a restored gospel: it has been prophecied and therefore will come to pass.
While this scripture does not give any way to reference a timescale as to when this might occurr: if one believes that a church that has existed since the time of Christ is true and is unchanged from that time, then one must, logically, accept that that church will fail, will change from true doctrine and lose its communion with God. The only other possible truth is that this has already happened, been covered up by those in authority at the time such that there appear to be records suggesting otherwise, and that the church is already in an apostate state. These are the only two possible, logical deductions that can be made from the prophecy of John, and LDS and Catholics are on diametrically opposing sides of the argument.