Many christian groups teach that if you do not continue to repent of each sin, you can lose out on the salvation. Rather than talk about that or compare how different faiths teach that (I'm sure someone else will do a great job answering that way) I would like to point out the one and only unforgivable sin.
Mark 3 (KJV)
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.
30 Because they said, He hath an unclean spirit.
Here we see that to "blaspheme against the Holy Ghost" will cause you to "hath never forgiveness"
This is said again in Matthew 12 (KJV)
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.
This is restated again in Luke 12:8-10 and in a less direct way in Hebrews 6:4-8 and Hebrews 10:26-29
I have talked to a couple of different faiths in the past and found that different faiths have different views of what it takes to blaspheme the Holy Ghost.
Catholics state that while it is an unforgivable sin, there is no sin that cannot be washed away by baptism and penance. So you can have this sin washed away. See Cathechism 1864
John Calvin believed that this sin could not be committed by believers. He came to this conclusion because of Hebrews 6:4-6. See Calvin’s Institutes of the Christian Religion Book III Chapter III Section 22
William W Combs summed most of these beliefs up here. He said that someone would have to have witnessed a sign miracle and then rejected the holy ghost, and he states that sign miracles no longer occur so today believer or not, you can't even commit the sin.
The general consensus of protestants that I have found so far that if you commit this sin, you are no longer able to repent.
The LDS church agrees with protestants about forgiveness not being possible, but they have specifically spelled out what it takes to commit this sin. It is very difficult for someone to commit it. If you do, (as far as I know) it is the only way someone living today can end up living with Satan in "outer darkness." Basically becoming a "son of perdition."
I have not been able to find the view of Orthodoxy churches on this. Specifically if they disagree with the Catholics or not.