Heber C. Kimball is quoted as saying the following in 1856:

The time will come when no man nor woman will be able to endure on borrowed light. Each will have to be guided by the light within himself. If you do not have it, how can you stand?

This quote seems to refer to some future test of the saints that would make it extremely difficult to remain in the faith unless one has a solid personal witness of the restored gospel.  J. Golden Kimball is quoted later in 1904 as saying the following:

I feel a good deal, or at least I imagine I do, like a man does when held up by a burglar and he is looking into the muzzle of a six-shooter. I would quietly and willingly hold my hands up, but during the time would think very profoundly of what I would do if given my liberty. We are in a similar position today, but all the men in the United States cannot prevent a man from thinking. There are not Apostles enough in the Church to prevent us from thinking, and they are not disposed to do so; but some people fancy because we have the Presidency and Apostles of the Church they will do the thinking for us. There are men and women so mentally lazy that they hardly think for themselves. To think calls for effort, which makes some men tired and wearies their souls. Now, brethren and sisters, we are surrounded with, such conditions that it requires not only thought, but the guidance of the Holy Spirit. Latter-day Saints, you must think for yourselves. No man or woman can remain in this Church on borrowed light. I am a strong believer in the following statement made by my father in the House of the Lord in 1856 “We think we are secure in the chambers of the everlasting hills, but the time will come when we will be so mixed up that it will be difficult to tell the face of a Saint from the face of an enemy to the people of God. Then, brethren, look out for the great sieve, for there will be a great sifting time, and many will fall; for I say unto you there is a test, a test, a TEST coming, and who will be able to stand?”

“Borrowed light” is normally interpreted to mean relying on someone else’s testimony of the gospel, so what kind of future “test” or “sieve” would impact a significant number of members of the Church in such an adverse way? Persecution? A scandal? An ultra-controversial change in policy? An institutional endorsement of controversial literature on Church history?

Your guesses?