No, as a general rule, it is not appropriate to accept a view solely due to lack of contrary evidence. This is a known fallacy, and it is called an argument from ignorance.
A Bit Longer Answer
Generally, what one must have to believe a proposition is an accumulation of evidence (or support) FOR the proposition, sufficient to justify belief in it, PLUS weak or no contrary evidence or justifications against it, PLUS sufficinet evidence or support against all the competing options to the proposition, such that the original proposition is more plausible than all of the other options.
But this is philosophy, so of course I am not done yet
Most atheists today try to avoid making an evidential case for atheism. There are a variety of rationalizations that I have seen, to try to avoid the work needed to do the bit longer answer above. I will discuss a few:
- One is the assertion that atheism is a NEGATIVE claim, and negative
claims do not need to be supported, only positive claims do.
However, this is an untenable assertion, one can craft any claim
linguistically to be positive or negative. Both positive and
negative claims are about our world, and claims about our world need
to be supported.
- Second, to supplement the first, is the claim that negatives can
never be "proven". This is silly too. Almost nothing can be
"proven" so there is a red herring in this assertion to start with
-- empiricism operates upon "support", not proof. And science supports or contradicts negatives all the time. Hypothesize an
effect, derive a predicted consequence, then test for the
consequence. If you don't find it, that is significant support for
a "negative" of the original claim.
- Third is the idea that the universe is too big to examine all of it
to find something which may be hiding. This is silly for most
claims -- they include details that specify where one would find
something, so look in those places. For a God claim, an example
would be where the God is a claimed Creator -- look at the claimed
creation for the tells of agency and intelligence in design. If
they aren't there, this is significant support for the negative of
the God claim.
- The claim, which you repeat here, that there is no evidence for any
God. This claim is blatantly false, and any even minimal study of
the beliefs of any religion would readily reveal this. Miracle
stories, recent miracles, mystics with personal experiences,
narratives about the extraordinary nature of the founder, etc are
all support -- evidence.
I will discuss later what these false rationalizations say about the atheists who espouse them.
I am not an atheist, but I know the basic rationale for the view:
- If one applies tests like in 3) to the top 5, or top 10 or so God
claims in the world, then I believe one can reasonably infer that
each of them has sufficient contradictions to reject them.
- This makes it plausible to infer that one is likely to be able to
eventually do this for all God claims, and it is therefore reasonable
to infer they are all significantly contradicted
- And there is an alternative ontology, that of physicalism, where
there would be no possible Gods, and this is a credible ontological
- And there are psychological reasons why people might delude
themselves into thinking there is/are God(s), which could explain the
widespread belief in them by what seem to otherwise be reasonably
competent and rational adults.
- The above list encompasses all of the alternatives to atheism.
The justification is generally longer when done with gusto, and will have other supporting elements as well, but the above is a pretty good summary, and MUCH stronger than the four invalid rationalizations I noted earlier.
Providing this, or a fleshed out stronger version of this case, would be a far more effective defense of atheism vs. the position of indeterminacy your friend was advocating for you.
Note you and your friend misused agnosticism. You two were applying three state logic (proposition justified, not sure of justification, proposition rejected) to the God Exists claim. But actual reasoning is four state (proposition justified, not sure of justification, preposition rejected, preposition incoherent or unevaluatable). For God Claims Agnosticism is the name for the fourth state, and indeterminacy or uncertainty is the second.
Again, this is philosophy, so no answer is simple and this one is still going.
As I noted, every religion cites justifications and evidences of their particular view. Many of these evidences lead to apparent contradictions between religions. This is bad for specific religions, but not necessarily bad for religion as a whole, and this is a major problem for the argument I listed for atheism. An OPEN MINDED theist could take those evidences, and say "yes, between the contradictions of evidences, and refutations of specific religious claims, none of the 5 or 10 or whatever religions examine carefully actually match this data fully. But this data supports that there IS some spiritual universe out there, with spiritual agents in it, even if the major religions' claims are flawed." This argument challenges the second bullet in the case I outlined.
The third bullet, physicalism, has a lot of problems to it, what with the Hard Problem of Consciousness, the growing consensus of the reality of abstract objects, and Hempel's Dilemma making physicalism undefinable.
The last -- can easily be reversed upon atheists. See the list of rationalizations I discussed at the outset. Atheists asserting rationalizations to support what they want to believe -- are as real and common a phenomenon as religious rationalizations. ALL humans are subject to confirmation bias and motivated reasoning. And every one of us should realize we may be subject to it in our core thinking.
I am not endorsing your friend's "indeterminacy" suggestion for you. This suggestion, that no atheists can credibly claim "knowledge" is one I have repeatedly seen from bad theist apologetics. The answer to those apologetics is the bullet list, I offered, or something similar. I assume that something like this bullet list is why you are an atheist, so lay it out there.
However, I AM suggesting that your reasons may not be as solid as you may imagine them to be, and the correct approach for a rational person, is to examine those assertions CRITICALLY, like an opponent would, rather than lovingly, like a motivated thinker would. Be sure you search for the hard cases, that might upend your thinking, rather than just for the reinforcing examples that will harden it.