Once a person truly believes in Christ they are born of God and cannot be unborn. *puts eternal security hat on now*. In that sense.. apostasy is falling away.. but not to a state of being unsaved after being saved (that is impossible,.. again *eternal security hat*).
Matt 25:31 - When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne.All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. He will put the sheep on his right and the goats on his left.
Notice this is referring to when Jesus comes back.. sheep and goats is not a general principle in the body of Christ. All those in God's church, all believers, are sheep! Yes, some sick sheep, and some almost dead sheep, some confused sheep who say they used to be sheep but now they don't believe they are sheep.. some sheep that don't even know they are a sheep.. but still sheep.. big difference between sick sheep and a goat. Sheep and goats refers to the people of the nations still alive after the tribulation period.. or at Christ's return (depending on if you believe in pre or mid or post trib or preterism etc etc). When the Lord speaks about giving water, visiting in prison, the sick etc.. He is talking about the events in the tribulation period.. we will need water, believers will be in prison,, and will be sick without medical care. Those gentiles who give assistance to Christians (and Jews) during the tribulation period.. they will be rewarded by Christ..they are sheep.. those who don't.. are goats. We need to stop interpreting everything Jesus says in terms of belief and works, salvation or no salvation, heaven or hell.. true or false.. it's in the realm of tree of knowledge of good and evil, try to use our spirit to interpret scripture rather than just our mind then you'll get a better understanding. I used to think sheep and goats were in the church, or is what happens in heaven.. but then my eyes were opened..it's talking about end-time events.
If we believe in pretrib rapture.. these sheep n goats are not Christian believers..they were already raptured... but good and bad gentiles. This is proved by the Lord judging them by their works and not by their faith in Him. If post trib .. well it's all the people, believers included, who survived the tribulation period. Their reward is to dwell under Christ's reign for 1000 years, and under the rule of the Christian believers, some who will be in charge of 5 cities, some 10 cities. etc.
27 says after death comes the judgement. There are no second chances after our death. 
