Okay, now lets look at those cases where Christians can choose to walk away.
Romans 11:19-24
19 You will say then, "Branches were broken off so that I might be grafted in." 20 Quite right, they were broken off for their unbelief, but you stand by your faith. Do not be conceited, but fear; 21 for if God did not spare the natural branches, neither will He spare you. 22 Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off. 23 And they also, if they do not continue in their unbelief, will be grafted in; for God is able to graft them in again. 24 For if you were cut off from what is by nature a wild olive tree, and were grafted contrary to nature into a cultivated olive tree, how much more shall these who are the natural branches be grafted into their own olive tree? NAS
This is only one of a plethora of warnings concerning those Christians (who are grafted into the natural olive) being cut off (because they walked away from faith), gathered, and burned.