$post object is null

I am trying to use the $post object from functions.php in my theme however if I attempt to var_dump($post) it returns NULL.

Here is my code:

function breadcrumb_navigation() {
    var_dump($post);
    $page = $post;
    $parents = array();
    while ($page->post_parent){
        array_push($parents, $page);
        $page = $page->post_parent;
    }
    if (sizeof($parents) > 0) {
        array_reverse($parents);
        foreach($parents as $parent) {
            echo '<li><a href="'.get_permalink($parent->ID).'">'.$parent->post_title.'</a><ul class="child">', wp_list_pages(array('child_of' => $post->post_parent,'exclude' => $post->ID)), '</ul></li>';
        }
    }
}

Can anyone tell why this would be happening?