I don't believe secularism can be said to be the primary cause of the two world wars and the holocaust as there were many factors, but I do think it played a role.
I think humans have a psychological need for certainty of truth, for hope in the future, for meaning and purpose, and for acceptance by their peers. All of these needs have more or less been met by religion in the past. When these needs are not met, this creates a very unstable situation where the first brilliant person with a lust for power and a talent for demagoguery can wreak havoc as he attempts to gratify his lusts.
I think secularism does create a vacuum in the minds of the populace and may prevent these needs from being met. Unless these people are taught to be extremely skeptical, this vacuum will suck in any doctrine that is fed them and they will rush to support this new "cause" as the highest ideal of humanity.
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