Why does declared indifference equal covert real zeal for the spread of slavery in other words why does Lincoln believe that congressional neutrality towards slavery actually furthers the spread of slavery

Basically its because inaction to it leads to its spread. According to Lincoln slavery in the US gives the right to others to call them hypocrites because the nation that is based of fundamental principle of freedom has the system of slavery. This system deprives America of its just influence and is in direct opposition to the founding principles laid out in the Declaration of Independence.