The difference of an (real) atheist from a believer is that we have no absolute certainties at all, especially about any god.
An atheist is uncertain only because:
None of the suppositions respecting the origin of the Universe are even conceivable in the true sense of the word. We think in relations, therefore by the laws of thought we are
1) prevented from forming a conception of absolute existence,
2) prevented from excluding the consciousness of absolute existence
From the necessity of thinking in relations,
it follows that the Relative is itself inconceivable, except as related to a real Non-relative. Unless a real Non-relative or Absolute be postulated, the Relative itself becomes absolute, and so brings the argument to a contradiction.
A "certain" belief is one which persists at all times and under all circumstances. The consciousness of absolute existence is a mental element which exists absolutely. If you don't understand this argument, you are in luck because I have excerpted some key parts of it below and provided a link to the full treatise. Happy thinking.
the vague consciousness which we retain of the objectively real, is of something which persists absolutely, under all changes of mode, form, or appearance. And the fact that we cannot form even an indefinite notion of the absolutely real, except as the absolutely persistent, implies that persistence is our ultimate test of the real whether as existing under its unknown form or under the form known to us.
Consequently, the result must be the same to us whether that which we perceive be the Unknowable itself, or an effect invariably wrought on us by the Unknowable.
Hence there may be drawn these conclusions: — First, that we have an indefinite consciousness of an absolute reality transcending relations, which is produced by the absolute persistence in us of something which survives all changes of relation. Second, that we have a definite consciousness of relative reality, which unceasingly persists in us under one or other of its forms, and under each form so long as the conditions of presentation are fulfilled; and that the relative reality, being thus continuously persistent in us, is as real to us as would be the absolute reality could it be immediately known. Third, that thought being possible only under relation, the relative reality can be conceived as such only in connexion with an absolute reality; and the connexion between the two being absolutely persistent in our consciousness, is real in the same sense as the terms it unites are real.
Thus then we may resume, with entire confidence, those realistic conceptions which Philosophy at first sight seems to dissipate. Though reality under the forms of our consciousness is but a conditioned effect of the absolute reality, yet this conditioned effect standing in indissoluble relation with its unconditioned cause, and being equally persistent with it so long as the conditions persist, is, to the consciousness supplying those conditions, equally real.
by taking away limits and conditions in successive acts, we form an indefinite notion of general existence. By fusing a series of states of consciousness, from each of which, as it arises, the limitations and conditions are abolished, there is produced a consciousness of something unconditioned. To speak more rigorously: — this consciousness is not the abstract of any one group of thoughts, ideas, or conceptions; but it is the abstract of all thoughts, ideas, or conceptions. That which is common to them all we predicate by the word existence. Dissociated as this becomes from each of its modes by the perpetual change of those modes, it remains as an indefinite consciousness of something constant under all modes — of being apart from its appearances. The distinction we feel between specialized existences and general existence, is the distinction between that which is changeable in us and that which is unchangeable. The contrast between the Absolute and the Relative in our minds, is really the contrast between that mental element which exists absolutely, and those which exist relatively.
So that this ultimate mental element is at once necessarily indefinite and necessarily indestructible. Our consciousness of the unconditioned being literally the unconditioned consciousness, or raw material of thought to which in thinking we give definite forms, it follows that an ever-present sense of real existence is the basis of our intelligence. As we can in successive mental acts get rid of all particular conditions and replace them by others, but cannot get rid of that undifferentiated substance of consciousness which is conditioned anew in every thought, there ever remains with us a sense of that which exists persistently and independently of conditions. While by the laws of thought we are prevented from forming a conception of absolute existence; we are by the laws of thought prevented from excluding the consciousness of absolute existence: this consciousness being, as we here see, the obverse of self-consciousness. And since the measure of relative validity among our beliefs, is the degree of their persistence in opposition to the efforts made to change them, it follows that this which persists at all times, under all circumstances, has the highest validity of any.
The points in this somewhat too elaborate argument are these: In the very assertion that all knowledge, properly so called, is Relative, there is involved the assertion that there exists a Non-relative. In each step of the argument by which this doctrine is established, the same assumption is made. From the necessity of thinking in relations, it follows that the Relative is itself inconceivable, except as related to a real Non-relative. Unless a real Non-relative or Absolute be postulated, the Relative itself becomes absolute, and so brings the argument to a contradiction. And on watching our thoughts we have seen how impossible it is to get rid of the consciousness of an Actuality lying behind Appearances; and how from this impossibility, results our indestructible belief in that Actuality.
