The Scriptures lay out some fairly stiff requirements for us to qualify as a person able to commit the unpardonable sin. It isn't just a case of having been baptised. In fact this isn't even given as a requirement. But, obviously, spiritual rebirth would be a requirement.
And if a person commits the unpardonable sin, what is their situation?
In regard the fornication that Alma's son, Corianton, committed, Alma said_
"Know you not, my son, that these things are an abomination in the sight of the Lord; yes, most abominable above all sins save it be the shedding of innocent blood or denying the Holy Ghost? For see, if you deny the Holy Ghost when it once has had place in you, and you know that you deny it, see, this is a sin which is unpardonable; yes, and whosoever murders against the light and knowledge of God, it is not easy for him to obtain forgiveness; yes, I say to you, my son, that it is not easy for him to obtain a forgiveness." Alma 39:5-6
This sets forth the idea that upon committing the unpardonable sin forgiveness has to be worked for, and is difficult. So why is it unpardonable if forgiveness can be obtained even if by difficulty?
"Wherefore I say to you, All manner of sin and blasphemy shall be forgiven to men: but the blasphemy against the Holy Ghost shall not be forgiven to men." Matt 12:31
This is setting forth that this blasphemy can't be forgiven at all. Why this apparent contradiction of statement? This is further brought up by Luke.
"And whosoever shall speak a word against the Son of man, it shall be forgiven him: but to him that blasphemes against the Holy Ghost it shall not be forgiven." Luke 12:10
The D&C also supports the idea that the sin can't be forgiven at all.
"The blasphemy against the Holy Ghost, which shall not be forgiven in the world nor out of the world, is in that you commit murder wherein you shed innocent blood, and assent to my death, after you have received my new and everlasting covenant, says the Lord God.." D&C 132:27
Paul appears to answer this apparent contradiction.
"For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again to repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame." Heb 6:4-6
In other words they can't have Christ suffer on behalf of their sins again. The Scriptures above are pointing out that while forgiveness can be worked at, Christ's atonement can't help. You have to do it yourself. This is mentioned in the D&C.
"For see, I, God, have suffered these things for all, that they might not suffer if they would repent; But if they would not repent they must suffer even as I." D&C 19:16-17
The big emphasis on being able to commit the unpardonable sin is the knowledge of the person. Note the following statement (about himself) by Paul _
"Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief." 1 Tim 1:13
Also note _
"For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered to them." 2 Pet 2:20-21
For those who make no effort to redeem themselves in any way from the situation of having committed the unpardonable sin we should note the following _
"Thus says the Lord concerning all those who know my power, and have been made partakers of it, and allowed themselves through the power of the devil to be overcome, and to deny the truth and defy my power-- They are they who are the sons of perdition, of whom I say that it had been better for them never to have been born; For they are vessels of wrath, doomed to suffer the wrath of God, with the devil and his angels in eternity; Concerning whom I have said there is no forgiveness in this world nor in the world to come-- Having denied the Holy Spirit after having received it, and having denied the Only Begotten Son of the Father, having crucified him to themselves and put him to an open shame. These are they who shall go away into the lake of fire and brimstone, with the devil and his angels-- And the only ones on whom the second death shall have any power; Yes, surely, the only ones who shall not be redeemed in the due time of the Lord, after the sufferings of his wrath. D&C 76:31-38
So, in summary, we see that there is quite a lot to actually committing the unpardonable sin. It isn't just a case of joining the church and then murdering someone. A great spiritual change is first required.
And this only makes sense. To rise high enough to gain eternal life we must gain a high level of knowledge and spirituality. To go low enough to go with the devil we must have an equal understanding and spirituality, to fall from. Or, as the saying goes, "the higher they are the harder they fall." Yet life is always better at the top (in spiritual things, anyway).