Firstly, I'd want to state two important things:
1) Christianity is a religion. Religion isn't a bad word, so we shouldn't try to separate Christianity from the term "religion"; but rather should recognize that real, true, and good religion can only be found in Jesus Christ.
2) Catholicism is Christianity. Just as Eastern and Oriental Orthodoxy are Christianity. Just as Lutheranism, Anglicanism, Methodism, Baptistism(?), Presbyterianism, etc are all Christianity too. Roman Catholics are Christians. Christianity, as a religion, encompasses a diverse number of denominations and theological traditions that all descend, in some way or another, from the original religion of Jesus' apostles. Which is closest to that, and how one determines what is the most faithful form of Christianity is kind of the whole argument everyone has and why everyone goes to their church rather than to another church. But it is critical to understand that as long as we are confessing the same core faith (as expressed in the Nicene Creed) we are Christians.
Back to the topic at hand:
I've never experienced "voices", and so I lack an experiential perspective; but I am inclined to assume that this could be psychological rather than demonic in nature in most cases. Not all cases, but I think there is both spiritual and psychological harm that can come by assuming the demonic before eliminating the psychological as cause.
With that said, I do not want to tell anyone that their experience isn't real or true. And so am going to proceed on assumption that we are dealing with the demonic.
In the Lutheran tradition there is actually quite a bit on the subject. Martin Luther was rather quite vocal about his own spiritual, psychological, and emotional struggles as a Christian. And regularly addressed the devil's lying tongue in his writings. One of the most famous things he had to write on the subject is actually in his most famous hymn, "A Mighty Fortress is Our God", here is the third stanza:
"And though this world, with devils filled,
should threaten to undo us,
we will not fear, for God has willed
His truth to triumph through us.
The prince of darkness grim,
we tremble not for him;
his rage we can endure,
for lo! his doom is sure;
one little word shall fell him."
Later on Luther specifically addresses what "one little word" means here. When Luther wrote "one little word", he was really saying that the tiniest, simplest, word can strike the devil down. So Luther writes that the one little word is "liar" or "devil, you lie". Calling the devil a liar to his face knocks him on his behind, strikes him down, and leaves him powerless. Recall what St. James writes in his letter,
"Resist the devil and he will flee." (James 4:7).
Our faith is in Christ, who has defeated the devil. The devil has already lost. The devil has already been defeated. The devil was defeated at Calvary, the devil was defeated when Jesus walked out of that tomb alive. Jesus ascended and sits at the right hand of the Father, and the devil has already been struck down and defeated--we have no need to fear him. The devil's power has been reduced to ashes, because Christ is Victor.
So we can endure the rage of the devil, Christ is with us.
So when the devil comes with his pack of lies, when he shoves your sins back in your face, stand firm in Christ, abide in Jesus and His word, tell the devil "devil, you lie".
To quote Luther,
"So when the devil throws your sins in your face and declares that you deserve death and hell, tell him this: 'I admit that I deserve death and hell, what of it? For I know One who suffered and made satisfaction on my behalf. His name is Jesus Christ, Son of God, and where He is there I shall be also!'"
Or as my pastor said in a discussion just this last week, "The devil is a lame-o".
For in Christ we are more than conquerors.
In Christ we have victory over sin, death, hell, and the devil.
In Christ all of our sins have been washed away.
In Christ we have been pardoned, and all debts canceled.
In Christ the power of death has been destroyed, and we have life eternal with Him.
In Christ the lies of the devil are reduced to pitiful whimpers.
In Christ the rage of the devil is mocked, for his doom is certain.
In Christ all the powers may rage and scream, but He holds the keys, He is Lord.
Christ has died, Christ is risen, Christ will come again. That is the victory of our God.
-CryptoLutheran