If your focus is confined to physical birth only, your point (A to B) is from birth to death on earth, which is altered to change your outlook on beliefs to; life only matters hear and now, to do what pleases you, and/or effects your life only. (A selfish outlook). Nothing else matters outside of your own life, or some choose to expand that to the lives of immediate friends and family, but usually not beyond that.
Take abortion for example, if your purpose is between physical birth and physical death, then life means only that much to you, so you can justify calling a baby a blob of tissue and do away with it as an inconvenience or the equivalent of a piece of trash. So justifying things in this light you can take your focus off of eternal consequences and put them on selfish decisions that only benefit you.
If there is an afterlife, or eternity,… that would mean that there would be consequences in what we do in the here and now, which would expand our outlook on life itself, tremendously. Now, that “tissue” is referred to as a baby that also has purpose because life now is catapulted way beyond these few years we spend on planet earth, into a never ending existence of perfection that was linked directly to what decisions we made before the physical death. So when existence is expanded into eternity then so is the meaning of life expanded outside of the walls of selfishness into, what is my now expanded purpose in life?
To bring as many people with us as possible.
What just happened?… Now your outlook is selfless and goes beyond yourself, friends and family to, now,… everybody.
In other words if, A to B means physical life to physical death, and C means eternal life then, A to B has only selfish purpose of what “you” want for “you”, and expanding it to C means that there is eternal purpose defined by God Himself of what He wants us to, selflessly do. Which is to give up this life, ( A to B, & live like this life is for you only)… to gain, ( C & live like this life is for God & others ) We, in turn, would live better, more peaceful lives here on earth as well as secure a never ending life of perfection with the creator of life Himself throughout eternity.
Matthew 10:39, & 16:25 He who finds his life will lose it and he who loses his life for His (Jesus) sake will find it.
If you switch from physical to eternal, the word purpose takes on an entirely new meaning.
No belief in God = your purpose is striving for a selfish, shallow existence but if your purpose stretches into eternity it never ends and goes way beyond you.