​Romans 11:21-22

  • For if God spared not the natural branches, [take heed] lest he also spare not thee. Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in [his] goodness: otherwise thou also shalt be cut off. (Romans 11:21-22)

These are verses often cited by Lordship Salvationist advocates to warn people of the dangerous prospect of losing their salvation, and, I would argue, usually to keep those people coming back to their teachings to learn how to maintain their salvation.  

I think some of the confusion comes about because, at least in the KJV, it's speaking in the singular in verses 21 and 22, so folks sometimes naturally assume it's talking to individual believers.  But the context shows a back-and-forth between singular and plural when discussing both the Gentiles and Israel in this chapter.
Israel (singular):
  • What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded (Romans 11:7)
Israel (plural):
  • (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day. (Romans 11:8)
The Gentiles (singular):
  • And if some of the branches be broken off, and thou, being a wild olive tree, wert graffed in among them, and with them partakest of the root and fatness of the olive tree; (Romans 11:17)
The Gentiles (plural):
  • For as ye in times past have not believed God, yet have now obtained mercy through their unbelief: (Romans 11:30)

The rest of the context makes it clear that verses 21 and 22 are talking about the Gentiles, not individual people:
  • I say then, Have they stumbled that they should fall? God forbid: but [rather] through their fall salvation [is come] unto the Gentiles, for to provoke them to jealousy. Now if the fall of them [be] the riches of the world, and the diminishing of them the riches of the Gentiles; how much more their fulness? For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office: (Romans 11:11-13)
  • For if the casting away of them [be] the reconciling of the world, what [shall] the receiving [of them be], but life from the dead? (Romans 11:15)

The second big thing that I see as potentially confusing is the present tense "if thou continue in [his] goodness" in verse 22.  Before I get to that, I want to make it clear that His goodness looks like it's tied directly to belief:
  • ​Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear: (Romans 11:20) 
  • And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again. (Romans 11:23) 
  • For as ye in times past have not believed God, yet have now obtained mercy through their unbelief: (Romans 11:30) 
  • Even so have these also now not believed, that through your mercy they also may obtain mercy. For God hath concluded them all in unbelief, that he might have mercy upon all. (Romans 11:31-32) 

If this is talking about Gentiles as a whole, and I think it's incredibly clear that Paul is, then what makes sense to me is that, as we approach the end of this world, we see a progressive corruption of the gospel and more and more people are not believing the real gospel.  For a start, we have fewer and fewer people even believing in God worldwide.  Statistics reflect a major decline in belief among the youth, something I, as a former atheist, participated in and can testify is indoctrinated into us in public education.  Even a generation ago prayer and Bible readings were still in schools.  I rarely meet people in my own age group who really believe the gospel, it's mostly people 10 or 20 years or older than I am that I come across, even on the internet, and most of them were saved when they were younger than I am.

Please consider how the Israelites were broken off of the olive tree, yet a remnant of them are saved:
  • ¶ I say then, Hath God cast away his people? God forbid. For I also am an Israelite, of the seed of Abraham, [of] the tribe of Benjamin. (Romans 11:1) 
  • Even so then at this present time also there is a remnant according to the election of grace. (Romans 11:5) 
In the same way, I think the same thing will apply to the Gentiles.  We Gentiles will have a small remnant of saved believers, but, as a whole, the branch of the Gentiles may be cut off in the coming years by works-based gospels and outright denial of God, if it isn't already in God's eyes.  That's our unbelief.  And thus the warning from God applies, beware, lest we also be cut off.  Don't fall for the increasing propagation of false gospels like Calvinism, Catholicism, Universalism, Pentacostalism, Ecumenism, and the like.  We need to corporately continue in His goodness and believe in His person and finished work, but individually we're saved forever after believing the gospel once.
Another perspective.