It is the quality of being based on or in accordance with reason or logic or being practical in the real sense.
This is what the quote says that we are neither atheist nor believer, rather we are or need to be rationale.
Whatever we like or find logical, we accept and what we see baseless, we leave, whether that is a thought or action or fellow human or even a religion.
जो तर्कसंगत लगे उसे अपनाएं बाकि को छोड आगे बढ जाऐं।
I also think that this is so true and simple, that removes all the unnecessary burden of social notions or relationships from our head.
Therefore to live this life beautifully and to make it meaningful we need to leave all these burdens and all the religion or relations are personal matters so they must not overcome our rationale thought process and practical actions.