If you were given a commission to make a road level, straight and smooth, how would you treat the following barriers?

Valleys–You would need to fill them in
Mountains/hills–You would need to bulldoze them and lower them to make them level
Crooked roads–you would need to straighten the roads
Rough roads–You would then need to smooth them out

Making a roadway level straight and smooth not only is possible but is done on a regular basis throughout the world.

A voice of one calling in the desert,
‘Prepare the way for the Lord,
make straight paths for him:

Every valley shall be filled in,
every mountain and hill made low.
The crooked roads shall become straight,
the rough ways smooth.
And all mankind will see God’s salvation.

Today the mandate continues.   

Make straight paths for him.

This is the work of the gospel.  Some have mountains that need leveled.  Some have valleys that need filled in.  Some have crooked roads that need straightening.  And some are rough roads needing smoothing.  But as we prepare the Lord, it is for this:

That all mankind will see God’s salvation.