On the mortality theme of your example "the six feet of earth make all men equal," here is a line from Thomas Gray's "Elegy Written in a Country Churchyard" (1751) that has become proverbial:

The paths of glory lead but to the grave.

The complete stanza reads:

The boast of heraldry, the pomp of pow'r,

 

And all that beauty, all that wealth e'er gave,

 

Awaits alike th' inevitable hour:

 

The paths of glory lead but to the grave.

(ll. 33-36)

On the mortality theme of your example "the six feet of earth make all men equal," here is a line from Thomas Gray's "Elegy Written in a Country Churchyard" (1751) that has become proverbial:

The paths of glory lead but to the grave.

The complete stanza reads:

The boast of heraldry, the pomp of pow'r,

And all that beauty, all that wealth e'er gave,

Awaits alike th' inevitable hour:

The paths of glory lead but to the grave.

(ll. 33-36)