If a man who has never heard the gospel will hearken to the teachings and manifestations of the Spirit of Christ, or the Light of Truth, which come to him, often spoken of as conscience—every man has a conscience and knows more or less when he does wrong, and the Spirit guides him if he will hearken to its whisperings—it will lead him eventually to the fulness of the gospel. That is, he is guided by the Light, and when the gospel comes he will be ready to receive it. This is what the Lord tells us in section 84 of the Doctrine and Covenants.
This Spirit of Truth, or Light of Christ, also has other functions. We read this in the revelation:
‘This … glory is that of the church of the Firstborn, even of God, the holiest of all, through Jesus Christ, his Son—He that ascended up on high, as also he descended below all things, in that he comprehended all things, that he might be in all and through all things, the light of truth; Which truth shineth. This is the light of Christ.’
(Joseph Fielding Smith, Doctrines of Salvation, 1:51)