OSAS stands for "Once Saved Always Saved." This is a common doctrine taught by many non-denominational churches, some Baptists, Provisionalists, and mainly Calvinist churches. It tends unfortunately to be the more mainstream view in evangelicalism today. What this doctrine suggests is that once someone puts their faith in Jesus Christ and genuinely receives the gospel and is born again, that this person can never backslide, fall away, or lose their salvation eternally. Scriptures used to back up this position include John 10:28, Romans 8:38-39, Philippians 1:6, and other scriptures that point to an assurance of salvation. However we should be careful, because the scriptures call us again and again to stand firm to the end, and to remain in Christ always.
And of course from Romans 11:19-22 ESV "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 fear. 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." Seems pretty cut and dry to me.
Is it really not worth having? If I tell someone that they can't lose their salvation, look at what I've removed: They can sin as much as they want, they can never lose it. No need for the fear of the Lord. No need for evangelism either, they can never lose it. In fact, they can ditch the body all together and go sit on the beach or watch tv, because hey, they can never lose it. When we say OSAS, we're cutting off the genuine motivation for believers to live like believers. So perhaps we think well, the motivation is love, not fear, right? Love is great yes, but more often than not we have to be motivated by a genuine reverent fear for the Lord. When we say OSAS, we remove all of that and more... and we wonder, why do we see so many Christians living just like the world, so many pastors addicted to pornography, so many lazy believers who don't evangelize? Well, we've taught a theology that leaves no urgency to keep living as a real Christian.
But why dispute over it? Does it really matter if someone can lose their salvation or not? It's worth having a discussion about it.
Yes, it's very important.
"Calvinists, who deny that salvation can ever be lost, reason on the subject in a marvelous way. They tell us, that no virgin's lamp can go out; no promising harvest be choked with thorns; no branch in Christ can ever be cut off from unfruitfulness; no pardon can ever be forfeited, and no name blotted out of God's book! They insist that no salt can ever lose its savor; nobody can ever "receive the grace of God in vain"; "bury his talents"; "neglect such great salvation"; trifle away "a day of grace"; "look back" after putting his hand to the gospel plow. Nobody can "grieve the Spirit" till He is "quenched," and strives no more, nor "deny the Lord that bought them"; nor "bring upon themselves swift destruction." Nobody, or body of believers, can ever get so lukewarm that Jesus will spew them out of His mouth.
-John Wesley on Once Saved Always Saved