Butch, the evidence is overwhelming. What you need to do is to trust the Lord Jesus as your only and sufficient Savior. Once you trust Him, you can know that you have salvation -- not a chance at salvation, but eternal life. He died for you and paid for your sins. He offers you His salvation. All He asks of you is to trust Him. Why can you not do that now?
All that the Father gives me will come to me, and whoever comes to me I will never cast out.
I give them eternal life, and they will never perish, and no one will snatch them out of my hand.
Kneel at the cross;
Christ will meet you there;
Come while He waits for you.
List' to His voice;
Leave with Him your care;
And begin life anew;
Kneel at the cross;
Leave cares behind;
Kneel at the cross;
Jesus will meet you there.
Come ye sinners,
Tired and weary,
Weak & wounded,
Sick and sore;
If you tarry till your better;
You will never come at all.
Not the righteous, not the righteous;
Sinners Jesus came to call.
If the evidence was overwhelming then you'd be able show where the Scriptures teach it, but you can't.
The passage you quoted from John 6 is speaking of the apostles. If you reads John 17 you'll see who they are.
6 I have manifested thy name unto
the men which thou gavest me out of the world: thine they were, and thou gavest them me; and they have kept thy word.
7 Now they have known that all things whatsoever thou hast given me are of thee.
8 For I have given unto them the words which thou gavest me; and they have received them, and have known surely that I came out from thee, and they have believed that thou didst send me.
9 I pray for them: I pray not for the world, but for
them which thou hast given me; for they are thine.
10 And all mine are thine, and thine are mine; and I am glorified in them.
11 And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name
those whom thou hast given me, that they may be one, as we are.
12 While I was with them in the world, I kept them in thy name:
those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled.
13 And now come I to thee; and these things I speak in the world, that they might have my joy fulfilled in themselves.
14 I have given them thy word; and the world hath hated them, because they are not of the world, even as I am not of the world.
15 I pray not that thou shouldest take them out of the world, but that thou shouldest keep them from the evil.
16 They are not of the world, even as I am not of the world.
17 Sanctify them through thy truth: thy word is truth.
18 As thou hast sent me into the world, even so have I also sent them into the world.
19 And for their sakes I sanctify myself, that they also might be sanctified through the truth. {sanctified...: or, truly sanctified}
20 Neither pray I for these alone, but for them also which shall believe on me through their word;
21 That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.
22 And the glory which thou gavest me I have given them; that they may be one, even as we are one:
23 I in them, and thou in me, that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me.
24 Father, I will that
they also, whom thou hast given me, be with me where I am; that they may behold my glory, which thou hast given me: for thou lovedst me before the foundation of the world. (Joh 17:6-24 KJV)
Notice Jesus used the past tense, those you gave me. He didn't say anything about anyone else being given to Him. However, Isaiah had already prophesied about this and says why they were given to Christ.
18 Behold, I and the children whom the LORD hath given me are for signs and for wonders in Israel from the LORD of hosts, which dwelleth in mount Zion. (Isa 8:18 KJV)
The verse from John 10 is in reference to the thief and says no one can snatch one out of God's hand. This passage doesn't address one leaving the faith.