There are a great many in this world who possess a faith which is so similar to a genuine saving faith that they themselves are deceived. Those around them may also consider this faith genuine even those who may have the gift of discernment.
There was a man named Simon Magus.
"Then Simon himself believed and was baptized. He began following Philip wherever he went, and he was amazed by the signs and great miracles Philip performed." (Acts 8:13 NLT).
His faith expressed itself in such a way that is was mistaken by Philip who considered him to be a Christian. But subsequently the Apostle Peter said:
"You can have no part in this, for your heart is not right with God… for I can see that you are full of bitter jealousy and are held captive by sin." (Acts 8:21, 23 NLT).
Arthur W. Pink, Practical Christianity (Public Domain) (paraphrase mine)
We forget that we are the proud owners of a reprobate mind (Romans 1). We are fully capable of creating a false salvation in the power of the flesh. But the truth remains that our righteousness is but filthy rags. We have lost our ability to do anything that pleases the Lord except that which is done by faith. So faith is the organ that the Lord must grow in our bosom until the innermost man is strong and the outer man is weak.
We are all infected and impure with sin. When we display our righteous deeds, they are nothing but filthy rags. Like autumn leaves, we wither and fall, and our sins sweep us away like the wind. (Isaiah 64:6 NLT)