Tell all the Truth but tell it slant -- 

To tell all the Truth can mean complete honesty. But to tell it slant can mean that truth is unreachable in itself or that in itself it is too much to handle. So that it must be told only from particular viewpoints or indirectly.
  
Success in Circuit lies

Circuit can be taken to be a social circuit. So Truth only matters if it becomes known within a group.


Too bright for our infirm Delight

The Truth is too much in itself for such a group's weak and sensitive sensibility.

The Truth's superb surprise 

The meaning or value of Truth has this surprising or shocking effect on people.

As Lighting to the Children eased

As the phenomenon of lighting is eased for children.

With explanation kind

And explained in baby-talk.

The Truth must dazzle gradually 

Truth must be seen gradually, perhaps as something accumulative.

Or every man be blind --  

Otherwise, no one will know the Truth.