Out of context again...no man is able to pluck them out of my Father's hand...
And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.  My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand. John 10:28-29 Pay attention to the context here. The "any man" and "no man" are outside of God's hand. The context of the "any man" and "no man" does not include the one in the hand as that is the "them". Those who are in the hand certainly can leave if they choose to do so. Scriptures absolutely supports this correct application of context. The prodigal son is a classic proof of this. The son was with the father, in His hand if you will, and then the son removed himself and went to a far country outside of the father's authority where the father could not care for him. We should know the rest of the story, the son "wasted his substance with riotous living" (Luke 15:13) and joined himself with someone other than his father. This is a clear picture of a child of God going back into sin, exactly what Ezekiel tells us this can happen and we should know that it does happen. When someone chooses to do this they will die spiritually. Again, When a righteous man doth turn from his righteousness, and commit iniquity, and I lay a stumblingblock before him, he shall die: because thou hast not given him warning, he shall die in his sin, and his righteousness which he hath done shall not be remembered; but his blood will I require at thine hand. Ezekiel 3:20 But we know that the prodigal son was able to repent and return to his Father and Ezekiel agrees with this as well. Nevertheless if thou warn the righteous man , that the righteous sin not, and he doth not sin, he shall surely live, because he is warned; also thou hast delivered thy soul. Ezekiel 3:21 Be careful to not believe the first lie told to mankind, that if you are saved and with the Father that you can never walk away from salvation, out from under God's care and authority. And the serpent said unto the woman, Ye shall not surely die...Genesis 3:4 We all should know what happened to the first two perfectly saved people who chose to disobey God. It is amazing how many people today are still accepting this lie. If you choose to leave God you certainly can but God has urged us not to so throughout His word. Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. Hebrews 3:12
The prodigal son parable is included here for ease of reference:
And he said, A certain man had two sons:  And the younger of them said to his father, Father, give me the portion of goods that falleth to me . And he divided unto them his living.  And not many days after the younger son gathered all together, and took his journey into a far country, and there wasted his substance with riotous living.  And when he had spent all, there arose a mighty famine in that land; and he began to be in want.  And he went and joined himself to a citizen of that country; and he sent him into his fields to feed swine.  And he would fain have filled his belly with the husks that the swine did eat: and no man gave unto him.  And when he came to himself, he said, How many hired servants of my father's have bread enough and to spare, and I perish with hunger!  I will arise and go to my father, and will say unto him, Father, I have sinned against heaven, and before thee,  And am no more worthy to be called thy son: make me as one of thy hired servants.  And he arose, and came to his father. But when he was yet a great way off, his father saw him, and had compassion, and ran, and fell on his neck, and kissed him.  And the son said unto him, Father, I have sinned against heaven, and in thy sight, and am no more worthy to be called thy son.  But the father said to his servants, Bring forth the best robe, and put it on him; and put a ring on his hand, and shoes on his feet:  And bring hither the fatted calf, and kill it ; and let us eat, and be merry:  For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry.  Now his elder son was in the field: and as he came and drew nigh to the house, he heard musick and dancing.  And he called one of the servants, and asked what these things meant.  And he said unto him, Thy brother is come; and thy father hath killed the fatted calf, because he hath received him safe and sound.  And he was angry, and would not go in: therefore came his father out, and intreated him.  And he answering said to his father, Lo, these many years do I serve thee, neither transgressed I at any time thy commandment: and yet thou never gavest me a kid, that I might make merry with my friends:  But as soon as this thy son was come, which hath devoured thy living with harlots, thou hast killed for him the fatted calf.  And he said unto him, Son, thou art ever with me, and all that I have is thine.  It was meet that we should make merry, and be glad: for this thy brother was dead, and is alive again; and was lost, and is found. Luke 15:11-32