The idea that there is an angel on one shoulder, and a devil on the other shoulder isn't actually that far from the truth.
The angel uses a still voice saying "Are you sure?", but the devil uses a shouting voice saying "DO IT, DO IT, DO IT".
Spiritually speaking what you hear then is something like this: "truth LIE truth LIE truth LIE...".
By the way, I'm not saying it's an actual voice, because I think it's more subtle than that.
Most people have learned to listen to the shouting voice, and have come to love their sinful life. They are subconsciously drowning out the voice of truth, and have stopped listening to it.
There is only one thing that can save you, and that is a love of the truth.
Even him, whose coming is after the working of Satan with all power and signs and lying wonders, And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
- 2 Thessalonians 2:9-10 (KJV)
A love of the truth will bring you to Jesus.
Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
- John 14:6 (KJV)
People do get saved from all of this of course, but the longer you're on this kind of path, the harder it will become.
In the end no one will ever be able to say "I didn't know", because everybody hears the still voice. It's just that we choose to obey the shouting voice instead.