Regards
I thought that's what you would quote. The problem is that this verse says nothing about what happens if we turn away from God, if we "jump" out of His hand. +John Chrysostomos in his VI Homily on Phillipians addresses this question:
"For, “woe unto you,” it is written, “through whom the name of God is blasphemed.” (Isa. lii. 5, LXX. nearly.) For if, when we have a son, (and what is there more our own than a son,) if therefore when we have a son, and are blasphemed through him, we publicly renounce him, turn away from him, and will not receive him; how much more will God, when He has ungrateful servants who blaspheme and insult Him, turn away from them and hate them? And who will take up him whom God hates and turns away from, but the Devil and the demons? And whomsoever the demons take, what hope of salvation is left for him? what consolation in life?
As long as we are in the hand of God, “no one is able to pluck us out” (John x. 28.), for that hand is strong; but when we fall away from that hand and that help, then are we lost, then are we exposed, ready to be snatched away, as a “bowing wall, and a tottering fence” (Ps. lxii. 3.); when the wall is weak, it will be easy for all to surmount. Think not this which I am about to say refers to Jerusalem alone, but to all men. And what was spoken of Jerusalem? “Now will I sing to my well-beloved a song of my beloved touching His vineyard. My well-beloved hath a vineyard in a very fruitful hill, and I made a fence about it, and surrounded it with a dike, and planted it with the vine of Sorech, and built a tower in the midst of it, and also dug a wine press in it, and I looked that it should bring forth grapes, and it brought forth thorns. And now, O men of Judah and inhabitants of Jerusalem, judge between Me and My vineyard. What should have been done to My vineyard, that I have not done to it? Wherefore, when I looked that it should bring forth grapes, brought it forth thorns? Now therefore I will tell you what I will do to My vineyard: I will take away the hedge thereof, and it shall be for a prey, and I will break down the wall thereof, and it shall be trodden down. And I will leave My vineyard, and it shall not be pruned or digged, but thorns shall come up upon it, as upon a desert land. I will also command the clouds, that they rain no rain upon it. For the vineyard of the Lord of Sabaoth is the house of Israel, and the men of Judah His pleasant plant. I looked that it should do judgment, but it did iniquity, and a cry instead of righteousness.” (Isa. v. 1–7, LXX.) This is spoken also of every soul. For when God who loveth man hath done all that is needful and man then bringeth forth thorns instead of grapes, He will take away the fence, and break down the wall, and we shall be for a prey. For hear what another prophet speaks in his lamentations: “Why hast thou broken down her fences, so that all they which pass by the way do pluck her? The boar out of the wood doth ravage it, and the wild beasts of the field feed on it.” (Ps. lxxx. 12, Ps. lxxx. 13.)...."
This is thw Faith of the Church. Your interpretation of +John is doubtless the basis of Luther's "sin boldly".
As others already noted, nothing in that excludes the possibility of falling to sin through one's own fault and therefore losing salvation. Not merely is it not stated, but if we were to impute such subtext in the Good Shepherd passage in John 10 we would break the metaphore carefully constructed by Christ.
I don't know if any of you ever raised sheep. No, I did not either, but this is what Christ is referring to. Shepherds would have a corral into which they would herd their sheep for the night, in order to protect them from predators and thieves. It had a single entrance, buit typically no gate. Instead a shepherd would lie down across the entrance and that way serve himself as a gate. Hence "I am the gate". Now, it would sometimes happen that several herds share the same corral, for example, near a marketplace. The shepherds would then have the task of separating their sheep from someone else's. In order to do so thay would trend their sheep to respond to a particular call. In the morning they would take turns calling their sheep, who would "know their master's voice" and come out of the corral.
There are two scenarios how a sheep may be "snatched": a predator or a thief may catch it, but the shepherd at the gate would prevent that; or a sheep may respond to the wrong shepherd and get out into the wrong shepherd's possession, but those who know their masters voice would not do that. But there is nothing in that system that prevents a loss of a sheep that mistakes a wrongdoer for its master, which is exactly analogous to sin committed by a free will.