It is firmly established that the elect were predestined from before the founding of the world. Both of our respective churches would agree with this foundation. I believe it is further established by Scripture that condemnation has also been decreed. However, indeed the chapters you cite must be considered alongside this doctrine.
John 15 is an excellent chapter for discourse. I suspect the reason for mentioning this chapter is to debunk the idea of perseverance. First, the relevant portions reads:
*I am the true vine, and My Father is the vinedresser.
"Every branch in Me that does not bear fruit, He takes away; and every branch that bears fruit, He prunes it so that it may bear more fruit.
"You are already clean because of the word which I have spoken to you.
"Abide in Me, and I in you. As the branch cannot bear fruit of itself unless it abides in the vine, so neither can you unless you abide in Me.
"I am the vine, you are the branches; he who abides in Me and I in him, he bears much fruit, for apart from Me you can do nothing.
"If anyone does not abide in Me, he is thrown away as a branch and dries up; and they gather them, and cast them into the fire and they are burned (John 15:1-6).*
Here’s Matthew Henry’s commentary, which I find useful in understanding this chapter:
*Jesus Christ is the Vine, the true Vine. The union of the human and Divine natures, and the fullness of the Spirit that is in him, resemble the root of the vine made fruitful by the moisture from a rich soil. Believers are branches of this Vine. The root is unseen, and our life is hid with Christ; the root bears the tree, diffuses sap to it, and in Christ are all supports and supplies. The branches of the vine are many, yet, meeting in the root, are all but one vine; thus all true Christians, though in place and opinion distant from each other, meet in Christ. Believers, like the branches of the vine, are weak, and unable to stand but as they are borne up. The Father is the Husbandman. Never was any husbandman so wise, so watchful, about his vineyard, as God is about his church, which therefore must prosper. We must be fruitful. From a vine we look for grapes, and from a Christian we look for a Christian temper, disposition, and life.
We must honour God, and do good; this is bearing fruit. The unfruitful are taken away. And even fruitful branches need pruning; for the best have notions, passions, and humours, that require to be taken away, which Christ has promised to forward the sanctification of believers, they will be thankful, for them. The word of Christ is spoken to all believers; and there is a cleansing virtue in that word, as it works grace, and works out corruption. And the more fruit we bring forth, the more we abound in what is good, the more our Lord is glorified. In order to fruitfulness, we must abide in Christ, must have union with him by faith. It is the great concern of all Christ’s disciples, constantly to keep up dependence upon Christ, and communion with him. True Christians find by experience, that any interruption in the exercise of their faith, causes holy affections to decline, their corruptions to revive, and their comforts to droop.
Those who abide not in Christ, though they may flourish for awhile in outward profession, yet come to nothing. The fire is the fittest place for withered branches; they are good for nothing else. Let us seek to live more simply on the fullness of Christ, and to grow more fruitful in every good word and work, so may our joy in Him and in his salvation be full.*
biblegateway.com/resources/commentaries/?action=getCommentaryText&cid=51&source=2&seq=i.50.15.1
With regard to Hebrews 6. First we must read this chapter along side the parable of the prodigal son. It cannot be that if a Christian sins then we are broken off with no hope. This is not the meaning of this chapter (although it is often abused in such a way). Also we are taught that if we do sin we may confess our sins or ask a brother (fellow Christian) to pray for us (see 1 John 1:8-10, 1 John 5:16-17).
Again, I think Henry does a wonderful job explaining this chapter:
*Every part of the truth and will of God should be set before all who profess the gospel, and be urged on their hearts and consciences. We should not be always speaking about outward things; these have their places and use, but often take up too much attention and time, which might be better employed. The humbled sinner who pleads guilty, and cries for mercy, can have no ground from this passage to be discouraged, whatever his conscience may accuse him of. Nor does it prove that any one who is made a new creature in Christ, ever becomes a final apostate from him. The apostle is not speaking of the falling away of mere professors, never convinced or influenced by the gospel. Such have nothing to fall away from, but an empty name, or hypocritical profession. Neither is he speaking of partial declinings or backslidings.
Nor are such sins meant, as Christians fall into through the strength of temptations, or the power of some worldly or fleshly lust. But the falling away here mentioned, is an open and avowed renouncing of Christ, from enmity of heart against him, his cause, and people, by men approving in their minds the deeds of his murderers, and all this after they have received the knowledge of the truth, and tasted some of its comforts. Of these it is said, that it is impossible to renew them again unto repentance.
Not because the blood of Christ is not sufficient to obtain pardon for this sin; but this sin, in its very nature, is opposite to repentance and every thing that leads to it. If those who through mistaken views of this passage, as well as of their own case, fear that there is no mercy for them, would attend to the account given of the nature of this sin, that it is a total and a willing renouncing of Christ, and his cause, and joining with his enemies, it would relieve them from wrong fears. We should ourselves beware, and caution others, of every approach near to a gulf so awful as apostasy; yet in doing this we should keep close to the word of God, and be careful not to wound and terrify the weak, or discourage the fallen and penitent. *
biblegateway.com/resources/commentaries/?action=getCommentaryText&cid=66&source=2&seq=i.65.6.1
Perseverance is generally the most tenuous doctrine espoused by John Calvin. However, it’s important to note, in the first instance, the methodology of this doctrine. It essentially traces backwards. It asserts that those who do not ultimately endure could not have been predestined to begin with. This is obviously true, since predestination was for all intents and purposes an eternal decree.
However, I suspect your biggest objection to this doctrine is the idea that the Christian can have eternal assurance. In other words you object to the idea that a Christian can profess faith but not yield fruit and still have an expectation of salvation – simply stated I object to this as well, and so did Calvin. This notion does great injustice to the doctrines of the Magisterial Reformers. However, none of the reformation churches (i.e. reformed or Lutheran denominations) hold this view – this is a contemporary manifestation of mass marketed Christianity by the Evangelical community (people like Joel Osteen, who IMO is a motivational speaker not a Pastor much less anything resembling a theologian).
It’s not as if these people are necessarily heretics, but I never hear them once discuss the consequences of sin or our duty as Christians. Essentially, it’s made for TV Christianity. Not worth the electricity you spend in watching it. I would rather watch EWTN any day of the week over that stuff if it’s any conciliation.