Some guy going under the name Chas_chas_123 thinks Richard Dawkins isn't sure on his "faith".

Seemingly unaware that atheism is not based on faith. Everyone is born an atheist, and everyone is an atheist in respect to most of the gods that we've ever dreamt up. Faith is choosing to believe in something, like a supreme intergalactic dictator, without evidence. Atheism is simply the default position towards theistic superstition.

Arch-atheist Richard Dawkins recently supported a campaign saying "There's probably no God. So stop worrying and enjoy your life"

Doesn't seem that he's very sure of his faith?
Is it rational to bet your life on a 'probably'?

"There's probably no God" refers to an advertising campaign run on buses in the UK, which Richard Dawkins supported, but the original idea was that of Ariane Sherine.

As an atheist myself, I cannot say for certain that there is no [insert your favourite god here]. Just like I cannot say there are definitely no Leprechauns, Langoliers or that we definitely do not live in a computer simulation. However based upon what we know about the universe the god hypothesis is unlikely. The specific god of certain Bronze Age beliefs from one tiny planet from one tiny period in time is even more unlikely.

No rational person would say that there is 100% definitely not anything, because a rationalist is always open to the possibility that something could be proven to exist given sufficient evidence.

Here's what Richard Dawkins had to say about the word probably in the slogan:

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I would like to add in addition to that, using the word probably helps atheists/rationalists/sceptics etc. differentiate themselves from absolutists like those of a religious background. You'll never see Christians use the slogan "There probably is a God, and there probably is a Hell for you to burn in eternity for, so you should probably go ahead and be a Christian".