David's sin in taking a census was due to the lack of dependence on God he was supposed to show.
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The Young David
In facing Goliath, David lived out his conviction that God was sovereign, and so his parent lack of strength vis-a-vis Goliath wasn't a problem. David lives out of his theological convictions.
In his old age, David has forgotten that God is the protector of his people. He enlists a huge army because he finds his security in a large army rather than in the power of God. David discounts his theological convictions by his actions.
David’s story is often our own. We say we believe God is my strength, but then we cultivate strength of our own to facet he challenges the world throws at us. Where does our security come from?