The Greek word used in the New Testament that is consistently translated as "repentance" is
metanoia. A super-literal translation of that word would be "over-mind". More accurately it is a change of mind, a change in thinking.
St. Paul wrote, "Do not be conformed to the pattern of this world, but be transformed by the renewing of your mind."
Jesus said, "The kingdom of God is near, repent and believe the good news."
There is a story concerning the Jewish historian Josephus, when he came across a Zealot (those who believed in using violence to overthrow Roman rule and bring peace to the Jewish people), Josephus told the young Zealot, "repent and follow me". What Josephus meant by this was "change how you think and do things my way".
In Hebrew there is the word teshuvah, again translated often into English as "repent"/"repentance", it literally means to return, or to turn around. That is one having strayed away by their sin, turns around and returns back from whence they came.
These ideas feature strongly in the Christian understanding of repentance: That repentance involves a recognition that we have erred, we have strayed--i.e. we have sinned--and that God wills that we be brought into, brought back into a state of reconciliation with Himself.
That this repentance, this change in our thinking means that we as sinners have fallen short, fallen astray, are not doing as we ought, failing to do what we should not; and that we confess this to God believing in His mercy by which He forgives us.
St. John in his first epistle writes, "If we confess our sins, God is faithful and just to forgive us our sins and cleanse us from all unrighteousness."
This makes repentance not a singular act, but rather a constant call to be recognize our waywardness, our failure to abide by God's will, and our need of mercy. This repentance draws us to confession of our sin, and to look to Christ to hold us, keep us, and save us.
* Do you have to keep repenting if you slip up and commit more sin?
It would be more accurate to say that repentance is a continual thing. It's not so much we repent, repent again, and then repent some more every time we screw up--we are always screwing up, sin is always present in what we do. So we are always called to repentance--to recognize our waywardness, our sin, and therefore to come humbly before God in confession--because God is always merciful, and His forgiveness is always present for us.
* If you fail to continue to repent.. is your salvation invalidated?
I would put it this way: Let's say you have a very good friend. Say you find yourself gossiping about that friend behind their back, or you start listening to people telling you rumors or gossips about that friend. And in various other ways, you start to betray the trust between your friend and yourself. Now, with each act of betrayal, you have the opportunity to go to your friend, talk about it, and because your friend is a good friend they will forgive you and keep loving you. But if you never talk it out with your friend, bit by bit you might find yourself becoming estranged from them, you spend less time with them, you start to have your doubts about them. Each little bit of betrayal, bit by bit, you are growing further from your friend. At what point have you killed the friendship from your end (not theirs, they still love you, and freely forgive you) and you start to act as though they aren't your friend at all?
Nobody "loses" their salvation because they mess up. But if we walk away from God, if we abandon Christ, and go and do things our way and refuse repentance, refuse to put our faith in God's mercy and love we make our way away from Him. That's the literal meaning of apostasy, from Greek
apostasia,
abandonment. To abandon, to leave, Christ, to leave our faith behind, to walk away and abandon God--that's apostasy.
-CryptoLutheran
