Matthew 12:31
31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
There are many sins that people do, but there are a handful that will cost your salvation, they include taking parts out of the book of revelation, joining satan after the 100 year reign, accepting the mark of the beast or worshipping the beast during the tribulation.
In this lifetime taking any part out of the book of revelation,
Revelation 22:19
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
Accepting the mark and worshipping the beast during the tribulation
Revelation 14:9–11
9 And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
10 The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
Joining Satan’s army after the 1000 year reign
Revelation 20:7–9
7 And when the thousand years are expired, Satan shall be loosed out of his prison,
8 And shall go out to deceive the nations which are in the four quarters of the earth, Gog, and Magog, to gather them together to battle: the number of whom is as the sand of the sea.
9 And they went up on the breadth of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them.
The blasphemy of the Holy Spirit is more than cursing him, but rather a perpetual hardening of the heart and willfully sinning against God and man (1 Timothy 4:2; Titus 1:15, Acts 7:51) In a nutshell the unpardonable sin is any sin that a person doesn’t want to give up, confess, or even ask forgiveness for and additionally doesn’t want to hear any more about it from the Holy Spirit.
Any sin mankind wants pardon for is forgivable, however, if we turn our backs on the voice of the Holy Spirit we begin to silence His convictions and eventually we cannot hear Him anymore. This effectively blocks the working of God in our lives because we have reached a point where we are unable respond to the promptings of the Holy Spirit, which places you in a place of being unforgivable.
Imagine you are tired of the the Holy Spirit convicting you of a particular sin. You want to completely remove His promptings even though the voice is warning you of danger and trying to show you the right path. So what do you do? You begin building a theoretical brick wall to block your conviction. Each brick in the wall represents a singular time you reject the conviction of the Holy Spirit. As you continue to reject the Holy Spirit, the “voice” you hear gets softer and softer. Eventually, as you lay more and more bricks down you cannot hear the Holy Spirit anymore, thus effectively cutting off your way of repentance and therefore salvation.