ACF Field value in wordpress login message filter

the_field() echoes out the result. You want get_field() instead.

However these won’t automatically default to reading fields from the home page. You’re going to have to give get_field the post ID of the home page to look up so it knows where to get the field from. Something like:

function alltomhobby_login_message( $message ) {
    if ( empty( $message ) ){
        $show_on_front = get_option( 'show_on_front' );
        $home_page_id = ( 'page' === $show_on_front )
                            ? (int) get_option( 'page_on_front' )
                            : ( ( 'post' === $show_on_front )
                                     ? (int) get_option( 'page_for_posts' ) : NULL );

        if ( isset( $home_page_id ) ) {
            $logintext = get_field( 'login_text', $home_page_id );
            return $logintext;
        }
    }

    return $message;
}

add_filter( 'login_message', 'alltomhobby_login_message' );

Or you could instead hard-code the page ID if you know it and it’s fixed.