The needful things regarding the Restored Gospel of Jesus Christ, are the basic, absolute and immutable truths. They are the rock upon which our lives should stand. For example a spiritual witness of, and faith in, the rock our redeemer, IS a foundation whereon if men build they cannot fall. (Helaman 5:12) That is a needful thing, and as the protestants correctly sing, “all other hope, is sinking sand!”
In a recent talk to the seminary and institute teachers Elder & Sister Renlund gave an analogy about a shipwrecked survivor floating aimlessly in the sea, and about to perish from lack of water and fatigue. Miraculously an old fishing boat sees them, plucks them out of the sea, and restores their hope. But as the boat slowly navigates toward land, the survivor begins to find fault with the boat, its captain, the water and food they provided. The faults begin to add up and those “many things” (which are meaningless in the face of the dramatic rescue) occupy the survivor's heart and mind, they simply cannot let them go. They fret, worry, become aggravated to the point that eventually they ask the captain to put them back in the water?
That parable explains what happens when we lose sight of the needful things and replace them with the many, trivial things.
For those being lead into personal apostasy the trivial things can be false histories, fake news, scientific discovery, philosophies of men, political corruption, obscure policies, contextual manipulations of true history, etc. Like the rusty boat, or stale water, that so disturbed the survivor, they can work on us until we forget that we were dead persons floating, and are now alive, saved. Thus we spin into apostasy troubled about many things that are, in comparison, meaningless.