But since you bring up OSAS, John 10:28 is a statement of the CAUSE and EFFECT of having eternal life.
When Jesus said "I give them eternal life" He was stating that He is the CAUSE of having eternal life. If He doesn't give it, one does not have it.
When Jesus said "and they shall never perish" He was stating that the EFFECT of having eternal life is never perishing.
So, the basis for never perishing is to HAVE eternal life.
So, instead of OSAS, it is really OGELSNP.
Or, once given eternal life, shall never perish. That's eternal security.
Part of the problem we seem to have here is that I give very clear verses in Matthew 10 and Luke and Acts about Judas being once saved obtaining part of the ministry and called a sheep by Jesus sent to the lost sheep and being one of "HIS" 12 apostles and you simple seem to direct away from the text and the clear meaning and pick another verse that you think applies against what Jesus said in Matthew 10. It is not either or, everything Jesus said can be harmonized. This is the way we must understand scripture. We cannot leave verses out of harmony and understanding with our doctrine because you might think they seem to contradict what Jesus said in another place.
I find no contradiction
espescially in John 10:28
this seems to be your main verse so we have to discuss it.
Jesus said in context
"9 I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture."(John 10:9 KJV)
Here we see that Jesus offers salvation to all men, and so all can be his sheep , saved sheep if they come in through him. Jesus already had said in chapter 5
John 6:47
"Verily, verily, I say unto you, He that believeth on me hath everlasting life."
So obviously belief and coming to Jesus with a humble heart after being convicted by Gods light in the heart and the ground broken up can receive the word as Jesus said about the seed. So the condition for having everlasting life ( which is Jesus Christ the life the eternal life 1 John 1:,1,2 etc etc) is to believe.
Then Jesus said in John 10
"14 I am the good shepherd, and know my sheep, and am known of mine."(John 10:14 KJV)
Jesus knew his sheep, because he knew all men, and he knows their hearts and what is in man. He is the true Light which lighteth every man that cometh into the world (John 1:9 KJV). This light is what men must come to and not hate (John 3:17-20 etc). if they hate the light they are condemned (John 3). And Jesus knew those who were coming to the light, those who had already believed in the light and those who hated the light and ran from it. So when he says his sheep hear his voice. That is
in their hearts they hear his voice For he has been calling men and drawing men and convicting men since the beginning of the creation of the world. Today of you hear his voice harden not your hearts.
So when he went about and called his sheep to him , if they were already saved and were looking for God’s righteousness they were filled with God and born again by the word of God. This made them his sheep. He knew them. So to say to Matthew come follow me and Matthew got up and followed him right away, showed that he was one of Hos sheep already. Just as the 12 apostles belonged to the father before they were given to Jesus and he knew his own and called his own to him. Here also Jesus knows his own sheep. He knows all men.
Now this doesn't mean for a second that the lost who listened to Jesus and were not his sheep did not have a chance to come to him and repent and believe. No he did offer that opportunity to every man and those who saw him espescially should have been able to see his good works and miracles and know and believe. And he had already said that any man can enter in in vs 9
Jesus also says in John 10
"25 Jesus answered them, I told you, and ye believed not: the works that I do in my Father's name, they bear witness of me. 26 But ye believe not, because ye are not of my sheep, as I said unto you. 27 My sheep hear my voice, and I know them, and they follow me: 28 And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand....38 But if I do, though ye believe not me, believe the works: that ye may know, and believe, that the Father is in me, and I in him..."42 And many believed on him there.""(John 10:25-28 KJV)
So in this understanding of knowing his sheep in their hearts we can see that Jesus speaks of those who abide in him and continue in his word as he said in John 15.
But notice in verse 25 that Jesus told them but they believed not. He offers truth to them and showed them many things. But they believed. If they were rejecting his word they are in a state of darkness. But this doesn't mean that they could not come to him still. He offers this to them in verse 38 where he said that ye may know, and believe"
So when Jesus says ye believe not because ye are not my sheep. They were not his sheep at that time or in eternal life at that time but we know that some of them who had not believed did believe because of his word as vs 42 "42 And many believed on him there.".
The opposite is true now when we get to vs 28 about Jesus giving his sheep eternal life. They are in that eternal life in Christ as long as they abide in him and continue in the faith grounded and settled and are not moved away from the hope of the gospel. They are in that eternal life which is to be in Christ and can not fall away as long as they abide and continue in belief. Jesus had already showed that the apostles could go away also from eternal life when he asked "will ye also go away".
Jesus calls all those that labour and are heavy laden. This labour and heavy laden is when they have come to the light and the light shines and breaks up the fallow ground of their heat and reproves all sin and they are exposed and humbled by God. If they receive him in their hearts and his word they have eternal life in them
But as we saw already in Johns other letter 1 John. He shows that a person can be walking in the light and say they are in the light then walk in darkness and try to say they are in the light and be a liar. They can say they know God but in works deny him. They can say they abide in him and yet walk in hatred towards their brother and so abide in death and darkness and no longer have eternal life ( Jesus Christ ) abiding in them
In John 10:28 Jesus is speaking of what he gives to those who continue n His words. Then and only then are they his disciples. If they abide in him and he in them and they continue in his word through faith.
Here are a few more conditions to be in John 10:28 and to continue in the faith.
Notice the conditional word "if"
John 8:31
"Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;"
"7 If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you."(John 15:7 KJV)
"10 If ye keep my commandments, ye shall abide in my love; even as I have kept my Father's commandments, and abide in his love."(John 15:10 KJV)
and back to John 10:28. They shall never perish. This is true as we abide in Christ. There is nothing that can separate us from Gods love towards us and his power to keep us. We are kept by the power of God THROUGH FAITH.
But if we deny the faith , cast off our first faith having damnation, lose faith, depart from the faith, turn from God depart from Him through and evil heart of unbelief. Then we cannot say we will not perish. God doesn't leave or forsake us , but men have that choice to do so.
We see a clear example of this in Judas and God says
2 Chronicles 15:2 KJV
"... The Lord is with you, while ye be with him; and if ye seek him, he will be found of you; but if ye forsake him, he will forsake you."
God doesn't leave first . But if we do then he does.
Jesus said clearly to all
"33 But whosoever shall deny me before men, him will I also deny before my Father which is in heaven."(Matthew 10:33 KJV)
So to understand John 10:28 you must understand who eternal life is, how to remain or abide in eternal life, and how do men no longer have eternal life in them. And the many conditions in scripture about salvation.
It is not by works of the law we are saved, or by our own works of the flesh or meritous works or sacraments etc. But By grace through faith. If we through faith have Christ dwell in our hearts by hearing the word and receiving then we have the spirit. But even if men start that way they can fall away from grace and have Christ profit them no more as we read in Galatians
"2 This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith?
3 Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh?
4 Have ye suffered so many things in vain? if it be yet in vain."(Galatians 3:2 KJV)
So we can see that they heard the word and received the Spirit by faith, then some went back into the works of the law and were in danger of this and by doing so their faith would be vain, or like a vapour of no value. Paul said if they did this, even though they did hear and receive the Spirit at one time that he stood n doubt of them and they were fallen from grace if they were doing this works of the law and seeking to be justified by it.
"19 My little children, of whom I travail in birth again until Christ be formed in you,
20 I desire to be present with you now, and to change my voice; for I stand in doubt of you"(Galatians 4:19,20 KJV)
Notice that they had to be born again again and to have Christ ( eternal life) formed in them again. Paul did not give them the OSAS talk, he said he stod in doubt of them. Does that sound like a OSAS preacher there? No.
"2 Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing....4 Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace."( Galatians 5:4 KJV)
As Jesus said, without him they can do NOTHING. To not have Christ profit them anything is to not be in salvation. To say Christ shall be of no effect unto them. Means that they are in danger in that sate and need to come back to Christ for salvation. To say they are fallen from grace. Means they are fallen from salvation, for by grace are we saved through faith.
Paul also mentions this about them in chapter one
"6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
9 As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed."(Galatians 1:6-7 KJV)