Is secular humanism to blame for our nation's breach into a post-Christian society?

First what is secular humanism?

Secular humanism is a humanist philosophy that upholds reason, ethics and justice, and specifically rejects the supernatural and the spiritual as the basis of moral reflection and decision-making. Like other types of humanism, secular humanism is a life stance focusing on the way human beings can lead good and happy lives.

How is it contributing to our society in a bad way?

Yes. In a number of ways! I will briefly go over a few. Here is a great chart!
Humanists usually do not believe that God even exists. Humanists believe mankind is the highest entity. (“Man is the measure of all things.”)
Humanism see man as basically good. Thinks that people should feel good about themselves regardless of their behavior. Tries to deal with guilt by positive self-talk.
Rejects the idea of a “sin nature.” Believes that whatever I want to do is ok, as long as it “doesn’t hurt anyone else.” (But is often shortsighted in deciding what may hurt someone else!) Tendency to rationalize that all behavior that I wish to do is acceptable.
Humanists believe some things are right for some people and some situations that may be “wrong” for other people and other situations. There is no absolute right and wrong. Everything depends on the situation.

Do you see how all of this can lead to post-modernism!

What does post-Christian mean? Are we there?

a post-Christian world is one where Christianity is no longer the dominant civil religion, but one that has, gradually over extended periods of time, assumed values, culture, and worldviews that are not necessarily Christian (and further may not necessarily reflect any world religion's standpoint).

America is leaning that way. I do believe.

Here is a good article discussing the differences in Christianity and Secular humanism and some great links.