Habakkuk 2:1-4
I will stand on my guard post And station myself on the rampart;
And I will keep watch to see what He will speak to me,
And how I may reply when I am reproved.
Then the Lord answered me and said,
“Record the vision And inscribe it on tablets,
That the one who reads it may run.
“For the vision is yet for the appointed time;
It hastens toward the goal and it will not fail.
Though it tarries, wait for it;
For it will certainly come, it will not delay.
“Behold, as for the proud one,
His soul is not right within him;
But the righteous will live by his faith.
Paul is quoting Habakkuk 2:4, which concludes a prophetic event whereby the watchmen is told to record his vision on tablets so that the one who reads it may run, i.e. escape the calamity. The prophecy will require an individual to make a life and death decision. Those who read the prophecy and believe it will take action to protect themselves and these will live. Those who who are proud will not take action and these will perish. God accounts those who had faith as justified.
Hopefully you can see from the context, this is not a passage about those who fell away after God declared them justified.
People talk about "salvation" as if it is some kind of metaphysical reality that we can't see. But we see from Habakkuk that faith resulted in actual, physical deliverance. We saw the same thing in the book of Exodus. The first born sons of those whose parents put blood on the door posts survived; the first born sons of those who failed to put the blood on the door posts perished.
No first born son who spent the night with blood on the door post perished.