Men that question the grounds by which Christians believe question it from a worldview that has often never been criticized. It seems like a big contradiction to me. I interpret the humanist as saying what is important to him, and I see it all as a vanity if he is right. If there is no God, then by what right, other than the one we give ourselves, do we have to criticize the way others live? In the end, we will all end up the same.
The question I would pose then, "Is life submit to our authority that we give it meaning or does life submit to one higher that we should be given meaning?" If the former, then isn't life only as important as each man gives it and not truly important in reality? If the latter, then are we not guilty of usurping God in the lives of others?
Sorry to snip your post but I just wanted to address a couple of you points.
It is absolutely true to say we are relatively insignificant when viewed on a galactic scale. The actions of one individual makes not a bit of difference when seen like that. But the key word here is 'relatively'. To each of us, our lives are hugely significant and what we do with them important. We extend this to those around us. Our families and friends, and those we easily empathise with. I believe this is the key that gives our lives a moral purpose. History has shown that it is easy for people to empathise with their own "kind". The Bible demonstrates this when viewed as a text that was written for ancient Hebrews. The ten commandments clearly only referred to their own 'kind'. As humanity has become aware of other peoples we have slowly expanded our 'kind' to include all of humanity (although admittedly there are many that are lagging in this regard).
Empathy is the foundation to all morals, I believe. I think it unfair to label this view as a vanity when it clearly (at least to me) isn't. It is about treating others the way we ourselves would want to be treated by others. These are the rules by which societies were founded. None can exist without it. Regardless of how we may disagree on the source of this, I'll bet that we agree on more than we disagree when it comes to moral principles. This in itself should tell you that a higher power is not the obvious answer as to why.
This is why I find your question a little strange. Life cannot be only as important as each man gives it because we don't live isolated from each other. We can only survive by cooperating. So life is as important as the collective society gives it. Individuals that make their own rules very quickly find themselves outcast.
Fine. But if you need the promise of everlasting life to have 'sound' moral principles, it seems to amount to the same thing,.