Once Saved Always Saved - What About Judas Iscariot?

Once saved, always saved. A great promise by our Lord.
I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand. (John 10:28-29 ESV)

However, from the perspective of the world, there are Christians who sinned and fall away, or change religion. Thus they conclude that Christians can lose their salvation.

This is a false conclusion. The apostle John told us that those who had fallen were not of us, else they would have continued with us. When they left, it proved that they did not belong with us.
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. (1 John 2:19 ESV)

Judas Iscariot was a perfect example. He was chosen and also not chosen.
Jesus answered them, "Did I not choose you, the twelve? And yet one of you is a devil." (John 6:70 ESV)

But from the beginning, long even before the birth of Jesus, it has been prophesied in the Psalms that one who ate bread together with Jesus would betray Him.
Even my close friend in whom I trusted, who ate my bread, has lifted his heel against me. (Psalm 41:9 ESV)

But Jesus knew whom He had chosen and Judas Iscariot was not among them.
I am not speaking of all of you; I know whom I have chosen. But the Scripture will be fulfilled, 'He who ate my bread has lifted his heel against me.' (John 13:18 ESV)

From the perspective of the world, Judas Iscariot had lost his salvation. But for us, he was never saved in the first place.

Once saved, always saved is not a false doctrine. It is a blessed assurance given by our great God.