Naturalism is a system of thought and action which denies the existence of God and instead believes everything happens according to scientifically explainable laws of nature. In this construct, then, human life exists only because of a process of evolution and its value is only determined by its usefulness.
We as Christians must reject naturalism because we know from Scripture that human life is designed by God and that humanity is very valuable to him. This is not to say we reject science; it is simply to say that God has ordained the laws of nature and they are under his purview.
Consider these verses from Scripture:
Then God said, “Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.” (Genesis 1:26).
This verse reminds us that humans are different from animals and have a special place in God’s creation.
For you formed my inward parts;
you knitted me together in my mother’s womb.
I praise you, for I am fearfully and wonderfully made.
Wonderful are your works;
my soul knows it very well.
My frame was not hidden from you,
when I was being made in secret,
intricately woven in the depths of the earth.
Your eyes saw my unformed substance;
in your book were written, every one of them,
the days that were formed for me,
when as yet there was none of them. (Psalm 139:13-16)
These verses remind us that God was intimately involved in our creation and that we did not just randomly come to be by some fluke of nature.
Before I formed you in the womb I knew you,
and before you were born I consecrated you. (Jeremiah 1:5)
This verse makes it clear that we were specifically designed by God with a purpose in mind to bring him glory.
In the end our worth is determined by Christ, who in love ransomed us from this sinful world and restored our relationship with our heavenly Father through his precious blood shed on the cross (I Corinthians 6:19-20). Only in him do we have life and have it abundantly (John 10:10).