Pardon me for butting into the discussion... :-) The OP's question was whether "once saved, always saved" is in the Bible. Paul here opposes that doctrine, and you seem to agree with him. Or have I misunderstood you?
Paul is saying that if you are depending on circumcision and keeping the Law, then you are no longer under grace. You have gone back to works which cannot save you.
What's happening in Galatians is that Judaizers are telling the new Christians they have to be circumcised and to keep the Law to be saved, which is not being under grace. Hence that would be a "falling away" from grace.
The people Paul are writing to are Gentile Christians who are saved and living under grace.
Paul is arguing that once you are saved under grace in Christ, you are always saved. If you appeal to anything other than grace for your salvation, such as works/good deeds to keep/earn your salvation then you have "fallen away" from grace. You are no longer relying upon Christ.
Hope that helps. If not, let me know.