What was the purpose of the document?
It had several purposes.
1st of all, to declare independence from the Great Britain.
Secondly, as it stated:
..a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
In other words, to explain WHY they wanted independence.
Secondly, to inform as to the rational of why they had the RIGHT to do so.
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness. That, to secure these rights, governments are instituted among men, deriving their just powers from the consent of the governed. That, whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it, and to institute new government, laying its foundation on such principles, and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness.
In other words, God has given every person liberty.
The Declaration went deeply into the entire Enlightenment's philosophy of God's direction for Mankind, and not just to separate from the British.