Once a person has received salvation they can't loose it.
John 10:27-29New King James Version (NKJV)
27 My sheep hear My voice, and I know them, and they follow Me. 28 And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand. 29 My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father’s hand.
		
		
	 
Baptists believe that a person must choose to believe in Christ in order to enter into a saving relationship with Him. Thereafter that status cannot be changed; not even if the person subsequently 
rejects the faith or falls back into a (
continual) lifestyle of sin. Thus, person can choose Christ UP UNTIL THE POINT of salvation. The same option (
apostasizing) is not available thereafter. Reformed Theology is more consistent in that it denies the freedom of human will both before, during and after the point where a person comes into right relationship with God. The doctrine of OSAS is the last remaining petal of Calvinism most types of Baptist still hold to. I will note in passing that this was NOT the view of the original Anabaptists who were persecuted by the Calvinists who were then in power. 
OSAS holds "eternal life" to be a 
static state either for 
legal or 
ontological reasons. I believe the the scripture teaches that salvation is a dynamic, ongoing,relationship of faith, faith meaning actively putting our trust in, confidence of reliance upon Christ and the truth of the Gospel.
In the verse you have cited here, the Apostle John uses 
present tense verbs which in Greek carries the sense of 
ongoing, continual activity.
[QUOTE
]The durative (linear or progressive) in the present stem: the action is represented as durative (in progress) and either as timeless (ἔστιν ὁ θεός) or as taking place in present time
http://www.ntgreek.net/present.htm[/QUOTE]
 Here I have added some words to emphasize this idea.
My sheep are hear
ing My voice, and I 
am know
ing them, and they 
are follow
ing Me. 28 And I 
am give (
am giving) them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand. 29 My Father, who has given them to Me, is greater than all; and
 no one is able to snatch them out of My Father’s hand. (
John 10:27-29)
1. The first thing Jesus says is that out of all those who were listening to His teaching 
certain ones were hearing (with understanding) and responding. Jesus said he 
was knowing them. There are several words for "know" in Greek. The one He uses here is 
ginosko which means to know by 
personal aquaintance or 
experience. Jesus was 
recognizing His sheep by their response to what He was saying.
2. The next thing Jesus said was that He was in the 
process of giving
ing eternal life to them.
3. They 
are follow (
ing) me
4. The word 
following means 
	
	
		
		
			to join one as a disciple, become or be his disciple
from the way disciples of that time "followed" their masters (physically)
http://biblehub.com/greek/190.htm
		 
In other words His sheep did not merely 
believe in Jesus they were following him 
as disciples, that is, they were 
obedient to His word.
5. The promise of eternal life, made to THESE followers was that they would 
never perish (
in hell) This verse is a 
promise to someone who had been (1)
 drawn to Jesus, (2) who have 
responded to the preaching of the Gospel and (3) 
are following Him. This verse is 
not addressing the case of a 
lapsed or 
apostate believer. 
To assume that a person 
solely on the basis of their initial response to the Gospel irrevocably unconditionally possesses eternal life is a presupposition read into the text. In fact the verse does not deal with the case of apostates or lapsed believers.
4. Although this issue is not addressed explicitly, it is implicitly. 
neither shall anyone 
snatch them out of My hand. 
The word "snatch" is 
harpazei according to 
Thayer's Lexicon means 
to seize, carry off by force 
This would be exemplified by a kidnapper, snatching a child out of his bed, restraining him and forcibly carrying him out of his parent's house. This cannot happen to believer who is following Christ but this example does not address the case of a child who runs away from home out of rebellion. When people depart from Christ it does not happen because 
they are overpowered and forced to do it but because they choose to do so. Once we enter the Father's house it is love and the inner working of the Spirit that keeps us there. The door is not bolted and locked. We choose to abide there because we need Him and love Him (
John 6:68).