If there is a sin for which one cannot be forgiven, then Christ did not die for their sins, because if He did, that sin would be included. Moreover, people are not condemned for any sin that they do, but because they have never believed on Christ.
Also, if faith is obedience, why is it distinguished from works in James 2? The faith in John 3:16 cannot be the faith in James 2, if faith is obedience. Why does it say faith without works is dead, if faith does not mean to simply believe?
James is a man looking on the outward appearance, where the evidence of our faith is in our manner of life. James says, YE SEE, how faith without works is dead, being alone. OTOH, God looketrh not on the outward appearance but on the heart.
There is no sin for which one cannot be forgiven up to a point. However, if one passes the time of God's open door, and the door gets closed, then he will no longer give light for salvation. There are many proofs for this and it takes place in the fullness of the time. In Matthew 12 Jesus pronounced a curse on the generation of Jewish rulers whose duty it was to recognize him and believe on him and lead the nation to faith in him. They did not and committed the unpardonable sin. Jesus said they would not be saved in this age, or the age to come.
2 Thess 2:7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:
9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
11 And for this cause God shall send them strong delusion, that they should believe a lie:
12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
Remember Noah's Ark. The door was closed before it began to rain.
The Hebrews in He 6 and 10.
One must be sure he is saved. "Behold, today is the day of salvation; now is the accepted time."
Moreover, people are not condemned for any sin that they do, but because they have never believed on Christ.
Not so. Here is the Great White Throne where the wicked will be judged and cast into the lake of fire. They are all judged out of the book of works. Their deeds. It is not what they did not do. It is what they did do.
Re 20:11 And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them.
12 And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.
13 And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.
14 And death and hell were cast into the lake of fire. This is the second death.
15 And whosoever was not found written in the book of life was cast into the lake of fire.
This is my understanding of the question at hand.