Deniehy extrapolated from history. He saw history as one moral improvement (and increasing reduction of tyranny). The natural end point of moral improvement over time is moral perfection. It is a pretty seductive and romantic view of humankind. Deniehy on the issue;

Few individuals who have traced the progress of society even from a semi-barbarous state to its present condition, will venture to deny that man is destined in this world to attain a state of moral perfection., in comparison with which the most refined and polished communities of ancient and modern times are sunk in the shade.

and;

The progress of events as viewed in the visible outside world around us, bespeaks an era of moral and social enjoyment, when an ordinary member of the community looking down through the sombre vista of time upon the philosophy of past ages, dimly shadowed in the distance, will exult in his destiny having placed him so far in advance of the wisest and greatest who preceded him.

So irresistible if the onward course of man in the march of improvement that even the trammels which the despotism of a northern dynasty has for centuries been weaving to enthral the human mind in a state of perpetuity, will at no distant day, snap asunder, and regenerated and intellectual man proclaim from the highest point of the Septentrion the triumph of a great social and moral revolution.

since mankind’s destiny is moral perfection

How can we assume that is the case?