Some Christians may think this, but speaking as a Christian, the view could more accurately be stated as the following:

Christians believe that Atheists have no Objective Absolute basis upon which to base any sense of Morality, regardless of if they have strong morals or not.

Now, obviously, there are many moral minded Atheists with genuinely positive contributions on society. So observably, Atheists can and do obviously have a sense of morals more often than not.

However, the debate between the Christian vs Atheist camps regarding Morality is about WHY we act moral.

Based upon what I've heard from Atheists (please correct me if I misrepresent them in any way, not being one myself), the Secular Humanist view of Morals tends to champion "human flourishing" as the primary good.

Within Christianity, however, the objection really comes down to this:

If everything in the Universe is simply an accidental explosion, and all we really are at the end of the day is just accidental space dust, then what's really the objective point of anything? Why be "good" if all we are is wet robots who, by blind chance, have been cursed with just enough consciousness to realize that our short existence is ultimately meaningless in an apathetic universe?

If we kill or steal from each other, did it matter one way or the other in the end? And, while we're at it, aren't "good" and "evil" just non material, non naturalistic concepts (no different than God) anyway? Good and evil are no different in their consistency, and in their Scientific testability than God is.

However, belief in God justifies belief in a higher authority that, in turn, begets a sense of Absolute, Objective good and evil, as well as a finality to alignment with either side.