I find they are merely deceived.
They've placed their faith in something as surely as we Christians have.
Of course they'll defend their beliefs just as strongly as us Christians.
It's up to the Holy Spirit to do the mind changing.
It's those times when they are NOT interacting with us that the real work is being accomplished.
There is already that internal doubt that their own writings and teachings produce. All we have to do is to show an alternative.
For instance; for those who have accepted LATER writings than the Bible or are awaiting even MORE revelation from GOD; they have to reconcile 1 Corinthians 1:7 where Paul states:
Therefore you do not lack any spiritual gift as you eagerly wait for our Lord Jesus Christ to be revealed.
There are two types of sinners in scriptures. One is like Eve who was deceived. The second type is like Adam who willfully understood and deliberately sinned against God. (1 Tim 2:14). It is not our place to judge and our hope is for salvation for all. But, at the same time, there are those who deliberately fight against the message. We are commanded not even to pray for those who would willfully speak against the work of the Holy Spirit to convict the world of sin, righteousness, and judgment. (John 16:8)
1 John 5:16-17 If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this.
They've placed their faith in something as surely as we Christians have. Of course they'll defend their beliefs just as strongly as us Christians.
Not so. My faith is built on the fact that I recognize that I'm a sinner and Christ Jesus paid my penalty before God. This faith is built on what is revealed in scripture composed by kings and paupers, people who rose to great heights and people who were martyred for their message. There are great miracles that confirms this message as well as eye witnesses that attest to the resurrected Living God. The Holy writings were written during a time when those around also experienced the same miracles and validated the writings. I also know this to be true because of work that Christ has done in me throughout my life. And whether God gives me riches or reduces me to poverty, it doesn't change the fact that He works all things to my good.
One can have faith that Baal is the ruler and jump around slashing themselves until the cows come home. It is a faith that is built upon the premise of what someone thinks is a good thing. They can give testimony of what their god had done for them like the women of Jeremiah who said that they had it good under the queen of heaven. They can even be a very noble person like Ghandi. Sure they might defend what they believe but it is built on nothing. It doesn't change anything internally or spiritually. They are simply old wine in new wine skins.