Hypothetically Yours

Dear Mr. Calvin,
In view of your position on the "perseverance of the saints"- could you please give your exegesis on Romans 11:21-23. According to your point of view, a believer cannot fall away and perish in hell. As I read in the afore mention Scripture reference; disobedient Israel, the chosen nation, had been disowned by God and gentiles adopted as a result. In verse 22 the gentiles are, in no ambiguous terms, warned to remain in God's kindness lest His severity be directed towards them and they be cut off{AFTER having been grafted in(verse 24)}. How can you reconcile your philosophy of the perseverance of the saints{once-saved-always-saved} in light of this Scripture reference?
--> Romans 11:21-23[&24]
21 For if God did not spare the natural branches, neither will he spare you.22 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.23 And even they, if they do not continue in their unbelief, will be grafted in, for God has the power to graft them in again.24 For if you were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these, the natural branches, be grafted back into their own olive tree.
-Hypothetically and in TRUTH, Jesus Follower