Calling a function from functions.php in custom page/ blog post

You could use add_shortcode if you want to use it within the editor.

function footag_func() {
    return "Test";
}
add_shortcode( 'footag', 'footag_func' );

And then use [footag] in your editor.

Or

Use code like this in functions.php and add a conditional tag

add_action( 'loop_start', 'your_function' );
function your_function() {
if ( is_singular('post') ) {
    echo 'Test';
    }
}

or

Create a function in functions.php

function your_function() {
return 'Test';

}

And then use this in your template

echo your_function();