In Romans 11:17-22 we are told about how gentiles are grafted into Israel to share in the blessings that God has promised through Abraham. Some think that it shows people being saved and then unsaved, so let's look at that passage and then review what it says.
Rom 11:17-22 But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. Then you will say, "Branches were broken off so that I might be grafted in." That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but stand in awe. For if God did not spare the natural branches, neither will he spare you. Note then the kindness and the severity of God: severity toward those who have fallen, but God's kindness to you, provided you continue in his kindness. Otherwise you too will be cut off.
Roman's chapter 11 is an interesting passage of scripture and how one understand it hinges on whether one considers that gentiles have been grafted into places left in 'Physical Israel' when 'Physical Israelite' branches were cut off, or whether one considers that gentiles converts have been grafted into 'Spiritual Israel' when 'Israelite Christians' were cut off the cultivated olive tree. Obviously they have to be grafted into a suitable 'matching' place.
On the one hand gentiles would have been grafted into physical Israel on being called, and thus become joint heirs of the promises. Now, having become heirs of Abraham they can accept Jesus (the greatest promise) and gain salvation. Then they move from physical Israel into 'Spiritual Israel'. If they reject so great a salvation then they can be cut off the cultivated olive tree on which they have been grafted. Not only that, if Israelites who were cut out of the 'physical' olive tree because of a lack of belief come to their senses, they can be grafted back in and if they not just believe but also accept Jesus as Lord for the first time they can gain salvation.
On the other hand, the second option is that Gentiles have been grafted into Spiritual Israel on being granted salvation and taken the place of Jewish Christians who had decided to reject Christ and refuse him as their Lord and saviour. In this scenario Gentiles who have accepted Christ as Saviour and Lord can reject him later and then be cut off from salvation. Also, Jews who once were Christians but who were cut off and lost their salvation can change their minds and come back and receive salvation again. God, not being a respector of persons, would of necessity have to allow Gentiles, who had salvation and then turned their backs on him, to return also and be granted salvation for the second time.
The question then becomes, 'which of the two possibilities above fits best with the other scriptures on salvation'?
We know from John 10:27-29 that once we give ourselves to Jesus Christ and place ourselves in his care by acepting his sacrifice on our behalf and making him our Lord and Saviour, then nothing can separate us from him, or shake us out of his hand.
John 10:27-29 “My sheep hear My voice, and I know them, and they follow Me; and I give unto them eternal life and they shall NEVER perish and no-one can snatch them out of my hand. My Father, who has given them to Me, is greater than all, and NO-ONE can snatch them out of My Father's hand.”
To me this and other similar scriptures makes the oscillating system a non-starter, because it makes God seem like a catcher with 'butter-fingers'. He has you and gives you eternal life and salvation and then he loses you and has to take them away. Then He gets another chance and gives you eternal life again and salvation but can lose you again....! So one has to ask how 'eternal' is eternal life? And your answer applies to God also!In the first letter of John we learn this...
1Joh 5:11-13 And this is the testimony, that God gave us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life. I write these things to you who believe in the name of the Son of God that you may know that you have eternal life.
John tells us that believers who put their trust in Jesus are, as with John, 'given' eternal life. It's something they 'have' already. How long does eternity last for? If God can give it to you one day and take it away the next, just how trustworthy is He when He gives us gifts?
But this scripture in the book of John fits well with the 'physical Israel' grafting system. Under this scenario if, say, Satan has hardened a person's heart and they do not believe Jesus is Lord they can be cut off and a gentile grafted in. Now if this gentile comes to understanding of the need for their sins to be forgiven and comes to belief in Jesus, this is not the same as salvation! Even the demons believe and tremble! They have also to accept Jesus' sacrifice and make him their Lord and Saviour. They have to put their trust in him. Then they are 'saved'!
If they do not put trust in Jesus, they can be cut off the cultivated olive tree. A jew who did not come to understanding and belief can be grafted back in to be given his/her opportunity to believe. God can soften the heart so that they come to understand and believe. But then they have to choose whether to accept or reject allegiance to Jesus. Do they follow the example of the demons who reject the sovereignty of Jesus or do they accept Christ's sovereignty, trust in him only, and receive salvation in Jesus Christ.
And to my understanding, once received always kept, as you live in Christ and grow in grace.
As children of Adam, our good deeds could not make us righteous;
the result of Adam's sin being so all encompassing. As sons of God,
our bad deeds cannot condemn us; the result of Jesus' blood shed
for us on Calvary being so all encompassing.
Best wishes for Grace and the Peace of Jesus.