The Means to (True) Life...

...is endurance in the faith, according to Christ at Luke 21.19. By your endurance you will gain your lives. Simple and to the point. Christ must not mean physical endurance, as if to say, if you endure like a marathon runner endures you will gain your lives, because then the Christian life would be about working out and who could become a contest of the physically fittest. We can rule that out because Paul said bodily exercise profits little. Christ must mean that by endurance in the faith you will gain your lives (or souls, or inner lives).

BDAG defines lives as seat and center of the inner human life in its many and varied aspects, soul and further as, the seat and center of life that transcends the earthly. Christ is then not speaking of physical life, but of eternal life. NET puts it most succinctly: By your endurance is a call to remain faithful, because trusting in Jesus is the means to life.