Secular Humanism is founded on the basic assumption that God does not exist and man, as an evolving animal, is perfectible. Thus, in opposition to irrevocable "natural law" (see Romans 1 & 2), the humanist accepts the capriciousness of human based rules. Evolution prohibits moral absolutes.
The US Declaration of Independence states, "We hold these truths to be self-evident, that all men are created equal; that they are endowed by their Creator with certain unalienable rights .". Evolution, as preach by secular humanists in public schools, cannot logically admit, "that all men are created equal" because for them there is no Creator and no equality among the survival of the fittest. When, during the punctuated equilibrium of evolution, did humanoids acquire unalienable rights? What gives a secular humanist any rights apart from his animal ancestors? As pragmatists they only rely on the results of situational ethics. Citizens are requested to obey government authority based, not on divine "natural law", but rather on legal positivism resulting from relative criterion.
Marxist Communism put the Proletariat State as the ultimate authority. As such, citizens were subject to arbitrary laws from the whims of Party Chairmen such as Lenin and Stalin, under whose rule 25 million citizens were sent to the prison camps of Siberia. Lenin set the stage for these atrocities by writing, "The revolutionary dictatorship of the proletariat is rule won and maintained by the use of violence by the proletariat against the bourgeoisie, rule that is unrestricted by any laws."
Another worldview that does not hold to an unchanging divine moral standard is the Cosmic Humanists of the New Age movement. Their premise is that since man is basically good then man can be a law unto himself. External laws only inhibit the self actualization of becoming One with the divine as each follows his god within.
Christianity, in stark contrast, recognizes the sinful nature of man. Law is a restraining gift from God. Left to follow his own inclinations, produces selfish hedonism and every imaginable vile evil. Legal positivists may make laws based upon the evolutionary needs of humanity. If, for example, hypothetical overpopulation threatens world food supply, the UN might call on nations to promote abortion and legalize homosexual marriages.
As a side note, it is curious that the United Nations is such a stronghold
for Cosmic Humanists. Robert Muller, author of New Genesis: Shaping a Global
Spirituality (1982), served as director of the UN budget and then director
of the office of secretary general in the early 70's. Julian Huxley, directed
the UN Ted Turner, signer and promoter of the Humanist Manifesto through his
broadcasting network of Fox TV and CBN, gave the UN over a billion dollars evidently
to bolster their shared worldview.