I want to expand a bit on the final statements that I made on Trust and Truth. One of the difficulties with having an external source of truth is that it must be independently verifiable, that it must be consistent, and that, as human language and society changes, it must find new ways of expressing the same truths. Giving that role to a Person, even if that Person is God Himself, requires either that He speaks the truth directly to every person, or that He works through other people to communicate that truth. I happen to think that He does a bit of both.
Now, for the first we must start with "Thick" moral realism, and believe that God created us with an innate sense of right and wrong which we can, through contemplation and rational thought, come to recognize and practice. While self-deception is possible, it never truly squashes that sense of right and wrong. Based on this understanding, God has directly communicated the truth to every person, which is why many pagans in the pre-Christian world would agree that a god or gods must be worshipped, elders should be honored; murder, stealing, and lying should be punished, etc. However, this innate sense is not enough.
For the second, we start with the person of Jesus Christ, who claimed to be "The way, The Truth, and the Life". Based on this and other sayings, it is clear that Jesus claimed to be God Himself. All these sayings are written in the Gospel books of the Bible, the authenticity of which is corroborated by a number of other sources. Jesus intended to found a church, saying to Simon "You are Rock and upon this Rock I will build my Church. " He also gave a teaching authority to all the apostles saying "Go, therefore, and make disciples of all nations". Jesus also spoke of the Holy Spirit coming to strengthen the apostles and guiding them in the truth after His death. With the selection of Matthias to replace Judas Iscariot in Acts, and the anointing and laying on of hands in the letter of the apostles, it is clear that the teaching authority of the apostles is handed on to the successors of the apostles. Based on this information, we can see that God does communicate the truth through other people, so long as they remain true to what was originally communicated by Christ to the Apostles and by the Apostles to their successors.
All this to say:
- We do have an innate sense of right and wrong (though we may deceive ourselves)
- The Church founded by Jesus Christ has the teaching authority needed to correct our self-deception and point us again towards the truth.