In Matthew, Mark, and Luke, the Lord mentions what has come to be called "the unpardonable sin", which is "blasphemy/speaking against the Holy Ghost":
Matthew 12:31-32
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.
32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.
Mark 3:28-29
28 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme:
29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.
Luke 12:10
10 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven.
What is this sin, can a Christian commit it, and if they do, do they lose their Salvation?
We don't have to speculate on what exactly this sin is, because it is explicitly stated what the sin was in the account in Mark:
Mark 3:22, 29-30
22 And the scribes which came down from Jerusalem said, He hath Beelzebub, and by the prince of the devils casteth he out devils.
29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.
30 Because they said, He hath an unclean spirit.
From above, we can see that the blasphemy against the Holy Ghost was saying that Jesus Christ was possessed by the Devil rather than indwelt by the Holy Spirit, and doing all His works through the Devil, while He was physically here on this planet.
Any other definition of this sin is based on speculation, extrapolating from what is actually stated in the accounts.
Does a Christian need to worry about committing this sin, either in the past, or potentially in the future?
Well, is Jesus Christ still on this planet, performing miracles? Can you see Christ do them? Can you then conclude, and then verbally declare that He is doing them through Satan? No, you can't. No one can.
Nowhere in Acts or the Epistles, where we are warned about such minor things as gossipping and being lazy, is this sin warned of, or even so much as mentioned. If this were something someone was still capable of doing, there is no indication of that anywhere in Scripture.
If someone had done this sin, the Holy Spirit would cease His work of convicting them entirely, and they would be permanently hardened by God. The person who did it would not care at all about having done it. The fact that someone would worry about doing this sin, is proof that they have not done this sin.
In summary, this sin was being fully convinced, and then verbally declaring, that Jesus was a false prophet who was demon-possessed, even after having seen Him, having heard Him speak, and having witnessed His miracles, while He was physically present here on earth. No living person, let alone a Christian, can commit this sin, and if they did, they would not care at all about having committed it.