Justify Your Existence -- at a glance this statement could mean either of the following.

1. Why am I alive?  Who put me here?  What's the meaning of life?
   
2. What do I do to make the world a better a place that justifies my presence in it?

The question was also posed with a request for references to non-religious texts.  So I'll assume the question was aimed at answering option #1.

I am an atheist.  This doesn't mean that I lack beliefs just that I don't believe a God exists.  This doesn't mean I am immoral or amoral; it means that I have had to create and justify my own moral code and accept the consequences as my own not the work of a higher being.  This also doesn't mean that I think life is pointless; it does mean that I must give my life purpose because there is no God or plan from God to give my life a sense of meaning.

To justify one's existence pre-supposes that there is a justification and/or that there needs to be a justification.  I reject this notion.  We are - period.  There is no justification and there doesn't need to be one.

Isn't that bleak?

To some the idea that there is no God is disturbing.  In my experience this is largely due to the individual's need to be part of something greater.  Part of God's plan.  The notion that we exist without purpose bothers most of the people I've met.

It bothers me that my life may be without purpose toward some greater goal.  However, that does not inspire to believe in God but rather to choose a greater purpose for my own life.

People don't like to believe that we are no more important than the family dog...

It is disconcerting to believe that we are no more important than the family dog or the rat which lives on our trash.  It is a blow to our ego to understand that bacteria do more to shape the Earth than we do.

Without God we have no excuses just responsibility.  We are not here to serve some higher purpose.  We are just here.  Evil is not born of Satan but of man.  Greatness is borne by God's providence but a man's good luck, hard work, and single minded focus.

Without God or a belief in something higher, there is no justification for existence and none is required.  As a result only option #2 (above) is valid.  And the answer to that question is simple:

We justify our own existence by our own actions.

The hard part is defining, for ourselves, what constitutues adequate jusitifcation.  The harder part is living up to our own standards.

My own Justification

Justification comes from action alone not intentions.

Justification does not come from the grand measures I take to feed the needy or cloth the homeless.  Rather it is my minute-by-minute actions that matter most.  The main pillars are:

1. Honesty
2. Integrity
3. Politeness/Kindness/Generosity
4. Contribution

All of these steps will demand courage at some point.  Sometimes I have that courage, sometimes I need to find that courage, and sometimes I fail.