There is the rational school of thought that relies on logical deduction for justifying knowledge. Then there is the empiricist school of thought that relies on sensory data induction for justification. The rational school of thought has largely failed, philosophically, because they think they can understand the universe with chains of logical deductions, but they ultimately fail and get debunked by the empirical school of thought - i.e. science.
The history of philosophy has exposed over and over that we can never really be as sure about anything as we might wish we are. The skepticism school of thought has evolved into postmodernism and has basically found ways of poking epistemic holes in everything. Any knowledge that you consider to be justified (in terms of 100% certainty) could be debunked by skeptical arguments.
So what this shows us is that the naïve desire for 100% certainty is unfounded, impractical, unrealistic, and impossible. This type of justification needs to be abolished and replaced by degrees of certainty, Bayesian probability, confidence intervals, and gradations of credence.
When you stop demanding 100% certainty that God doesn't exist, and instead try to calculate the probability of God, you will find that it is very close to 0%. We all naturally agree that we know things when probabilities are very close to 100% or 0%. Therefore, we can be justified in knowing that there is no God.