Most people in the modern world grow up in households in which the prevailing moral ideas are shaped by a type of secular humanism which might appear not too different to Christianity if it were a religion purely of this world with no sense of the next*. That’s not surprising since secular humanism has borrowed much of its moral position from Christianity, but it leads many people to think that secular humanism has taken all the important bits of Christianity and got rid of the superfluous stuff. The truth is precisely the opposite.
What is left out is the reality of the absolute and this is the foundation of all the rest. Without the fatherhood of God the brotherhood of man has no meaning, and the undoubted moral falling away in the West and elsewhere comes from replacing a morality rooted in spiritual truth with a secular morality that has no roots in anything real. The result is that each generation becomes further separated from the idea of spiritual truth and, as it does, its sense of a genuine moral order, founded on a real transcendent truth is diminished.
Once you start to separate human intellect and will and feelings from their proper source in spiritual realities you lose the all important connection between the human being as he is here in this world and his true origin and purpose which is in higher dimensions of existence.
For the fact is that the basis for an objective standard of morality can only come from something eternal that is outside the human mind. This is God and it must be a personal God too because impersonal reality is just that, impersonal. An impersonal reality would be truth only but a full and complete morality must be based on love and truth equally and that means a personal God. Only if founded on a transcendent reality can morality itself be real in the sense of being the same at all times and in all places and not contingent on external factors. But any morality that comes from the human mind will inevitably be conditional and dependent on circumstances affecting that mind. In consequence it will be both mutable and relative.
Secular morality denies religion so it has to look elsewhere to justify itself. Often nowadays it looks to science but science can only examine the realm of nature and nature cannot explain itself so science has nothing to say about true morality. This is an inversion of truth in which the tail is seen to be wagging the dog. No wonder it get everything back to front. And when it comes down to it the very phrase 'secular morality' is a contradiction in terms since secular means something of this world while morality if it is to be grounded in anything real necessarily implies something beyond this world, something that takes its origin from a timeless reality of truth and goodness. If there is not this reality then any system of behaviour is arbitrary and meaningless and one is not better than another from an absolute point of view. This leaves you with a morality that can change at any time and a changing morality is no morality at all.
The point I am making here does not mean that an atheist can never act from a moral position. Of course he can but what I am saying is that if he does, and that position coincides with what is generally accepted as goodness, he can have no real reason for doing so according to his belief of how reality is structured. Indeed, without some kind of sense of an absolute that overrides everything in this world and against which everything in this world must be measured how can the atheist actually define anything as 'good' as such? His belief, logically considered, must mean that there is no real moral difference between a Hitler and a saint. Each is just pursuing his own subjective preference and there is no real reason to say that one is better or worse than the other. Against what true yardstick would you be measuring this? The common good? But there is no common good if all we are is animated matter. Without an over-arching reality we are all just locked in our little selves with no true connection to anybody else. Consequently there is no real reason for me to care about you except for any advantage it might bring me. And that is why I say there can be no such thing as secular morality.
* Which a lot of contemporary Christianity is becoming but that's another story.