We really can't use "life" as the word here.

Eggs are alive. Sperm are alive. The fertilized zygote is alive. It's all alive, all the time; indeed, two alive things fuse into one alive thing during fertilization.

If by "life" you don't mean "being alive", you should use a different term. (The religious idea seems descended from the idea of vitalism where there was actually a moment where life began--or maybe there was just the divine vital spark which flowed through life. Anyway, ensoulment was generally considered to be a separate thing (not equivalent to life).)