How do reformed theologians reconcile John 15:2 with “once saved always saved"?

Reformed Christians believe that fruit ("good works") are an inevitable consequence of faith. One consequence of faith is salvation. The other, fruit. There is no such thing as a "believer" that does not bear fruit. The statement is redundant, as stated by Jesus in verse 5 of the same passage:

he who abides in Me and I in him, he bears much fruit

The ones that are in Him will not be cut off, because they bear fruit, because of the work of God in them who is the author and finisher of our faith.

When Jesus speaks about branches "in Him" that do not bear fruit, he must be talking about "in Him" in a different sense than the way that he guarantees to bear fruit, otherwise he would be contradicting Himself.

This other, unfruitful way to be in Christ is generally believed to be part of the Church outwardly, but not have saving faith. The ones that are in Christ in truth, by faith, will bear fruit - which is not a condition of salvation, but a consequence.

Ezekiel chapter 18 would be a better reference to contradict the teaching of OSAS.

One is saved as long as you remain faithful to God. Choosing your destiny is a personal choice, and one that you must make daily.

The biblical teaching on salvation is better expressed as “once saved always saved as long as you stay saved”. ie, as long as you abide in him.

The OSAS philosophy is closely linked to the teaching of predestination. Predestination is the teaching that God chooses, or has already chosen who will and who will not be saved.

If you are not saved, it is because God did not choose you. In other words, it is not your fault, it is God’s fault. As soon as you head down this path you realise exactly who it is that originally blamed God, and that if you follow this teaching you are following him and not God.

Man has been given free will. Each is to choose his own destiny, or as Paul put it, work out your own salvation with fear and trembling.

Speaking of himself Paul also said “Not that I have already attained, or am already perfected; but I press on, that I may lay hold of that for which Christ Jesus has also laid hold of me.”

If Paul does not take his salvation for granted, but picks up his cross daily, and strives that “if, by any means, I may attain to the resurrection from the dead”, then we must do the same.

Frequently the idea that you are not saved unless you remain saved is spoken against as being false because then you have no joy in assurance of your salvation. This is like the Pharisee who prayed loudly to God looking up to heaven and thanking him for his salvation, and that he was not like the publican. The publican however beat his chest and would not even look up to heaven and prayed, “God, forgive me for I am a sinner.”

The prayer of the publican has always been unpopular, and is becoming even more unpopular of late.

There are understood to be two different ways of being "in the vine," and only one is a reference to true salvation.

The illustration in question clearly describes (in addition to branches which are fully a living part of the tree) branches which are outwardly, visibly attached to the tree, but which do not inwardly share in the life of the tree. They're still dead.

Reformed theologians understand that entire breadth of concept to carry over to the spiritual things it's illustrating. There are those who are outwardly a part of the Church (the "Body of Christ" in 1 Corinthians 12), but who have never experienced the inward change that produces spiritual life. They look saved, but they never really were.

This warning, then, like the warning in Hebrews 6, is really a warning to the visible church to be sure they're really saved and not just playing a part.

Reformed Pastor John Piper addresses this:

https://www.desiringgod.org/interviews/does-john-15-defy-calvinism

He claims that there are two forms of attachment to Jesus. One is a saving attachment and the other is not. He cites Judas as an example of someone who had an attachment to Jesus that was not a saving attachment. Judas went out like the other disciples and cast out demons and healed people in the name of Jesus. He could not have done those things if he had no attachment to Christ and his power. Even so, he was cut off. Then in John 17, during the last supper, only a short time after he sent Judas away, he says:

While I was with them, I kept them in your name, which you have given me. I have guarded them, and not one of them has been lost except the son of destruction, that the Scripture might be fulfilled. (John 17:12)

If there was one son of destruction, there are many. They have a connection to Christ, but not the permanent one Jesus affirms that the other disciples share.

1. It is true that Jesus tells his disciples that they are branches in him, the vine. However, he nowhere says that all the branches in the vine are his disciples.

2. He nowhere says that any disciples “will be taken away and burned.”

3. Then who are the branches that will be taken away? Why are they removed? They don’t bear fruit.

4. Why don’t they bear fruit? Because they don’t “abide” in the vine.

5. Who are those who abide in the vine and bear fruit? His disciples (v8). It is the fruit that proves they are disciples, learning from him.

6. Therefore, the fruitless branches, the branches that are removed and burned, cannot be his disciples, since they don’t abide in him and so learn from him.

Just the idea of remaining should clarify that a person can be once there on the vine but be pruned off, because they themselves rejected what they formerly had and didn't produce fruit.

Matthew 7:22-23 seems to say clearly that even people who call Jesus Lord Lord, and who cast out demons in his name and do miracles in his name are not recognized as being "saved."

22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.

This scripture insinuates that they were never saved in the first place, but it strongly seems to say that they were believers in Jesus and believed they were saved and called on his name to cast out demons which fogs up a bit "Whoever believeth and is baptized shall be saved."

The whole vine analogy loses a lot when separated by centuries of non-agricultural life. This is not primarily a scientific description of salvation but a spiritual description of the kingdom of heaven. A vine has branches that bud and bear not just leaves but fruit also and it has other branches that bear leaves only. These are colloquially called 'suckers' nowadays and they are routinely pruned off not just because they do not produce fruit but because they use up energy that is more profitably sent to the fruit bearing branches and also because they provide to the fruit bearing branches an overabundance of shade from the life-giving sun.

The impact of excessive shading

Many vine training systems are designed to avoid excessive shading of the fruit by the leafy growth (the "canopy"). While some shading is beneficial, especially in very hot and sunny climates, to prevent heat stress, excessive amounts of shading can have negative impact on grape development. As a photosynthetic plant, grapevines need access to sunlight in order to complete their physiological processes.[5] Through photosynthesis, less than 10% of the full sunlight received by a leaf is converted into energy which makes obstacles such as shading even more detrimental to the plant. Even if the leaves at the top of the canopy are receiving plenty of sunlight, the young buds, grape clusters and leaves below will still experience some negative impact. During the annual growth cycle of the grapevine, excessive shading can reduce the success rate of bud formation, budbreak, fruit set as well as the size and quantity of grape berries on a cluster.2

The grape clusters receive some benefit from receiving direct sunlight through enhanced ripening of various phenolic compounds that can contribute to a wine's aroma and quality. In addition to having decreased physiological ripeness, excessive shade will negatively impact a grape's quality by causing increases in the levels of potassium, malic acid and pH in the grapes while decreasing the amount of sugar, tartaric acid and color producing anthocyanins. Beyond a lack of sunlight, excessive shading limits the amount of air circulation that can take place within a vine's canopy. In wet, humid climates poor air circulation can promote the development of various grape diseases such as powdery mildew and grey rot.2

The suckers are in the vine physically in the same way that unbelievers can be within the visible church and partake of some benefit thereby while at the same time sapping it's resources and energy. They are in the vine metaphorically in the same way that the Tares are among the Wheat. And yet they have no part in the true vine (Christ) spiritually in much the same way that

not all who are descended from Israel belong to Israel - Romans 9:6

Our primary (and very difficult) responsibility is examine ourselves to see whether we are in the faith and then to allow metaphor, analogy, correlation, statement and parable to simply be what they are without over or under spiritualizing everything and to ensure that our conclusions harmonize with the overall testimony of Scripture.

Within that sentence are two statements. The first is that the Father (who is the vine-dresser) will take away the branch that does not bear fruit. The second is that the Father will prune the branch that does bear fruit so that it may bear more fruit.

If this parable was the only place in the New Testament to address the issue of eternal security, then there would be every reason to fear that a lack of “fruit” would result in the loss of salvation. But it isn’t. Please read John 6:39-40:

“And this is the will of him who sent me that I should lose nothing of all that he has given me, but raise it up on the last day. For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day.”

Given that John 15 is a metaphor or an allegory, then we do well to consider it in the light of other passages that speak clearly about the assurance of salvation given to those who have come to saving faith in Christ Jesus. It would be foolish to take three verses alone to support a view, one way or the other.

Isaiah 5:1-7 is a striking Old Testament image of Israel being likened to an unproductive vineyard that is rejected by God and made into a wasteland. Here, God is rejecting his people because they have rejected his law. No doubt, Jesus’ audience were familiar with the illustration. But God has never entirely abandoned his people, and even in the future, all of Israel will be reconciled to God. Yes, there is a dire warning against any person who claims to belong to God yet proves by their actions (or lack of actions) they have only an outward profession of faith.

In Matthew 13 Jesus gave a parable about seed that fell into different types of soil. Some plants start to grow but then wither away. Does that mean the person who has been adopted into God's family has no security, that they must constantly strive to do good works? No. If a branch is not receiving the life-giving nourishment it needs from the roots, then it will not produce fruit. If a person is not genuinely abiding in Christ and living their life to the glory of God but is simply pretending to be a Christian, then they too will wither away like the seed that fell onto stony ground. Jesus will say to them, “I never knew you.” If a person does not obey Jesus then they are not a part of the one, true vine.

You ask, does John 15:2 disprove “once saved always saved?” Frankly, that expression is so misleading I reject it because it gives entirely the wrong idea about the grace of God and the doctrine of salvation. I put it to you that John 15:2 neither proves nor disproves any false assumption that a person can make a claim to be a Christian without producing evidence of a transformed life, a life that produces the fruit of the Spirit. Only those who are indwelt by the Spirit will mature and grow in their Christian walk, producing the fruit that brings honour to our Lord and Saviour. All I can say is it’s a good job Jesus knows who belong to him.

EDIT: Given the extended and currently revived comments that follow my answer, can someone please place these comments into a chat room? I don't know how to do that. Thanks.