Kalimah Tayyibah = La ilaha ila LLah
(I may have transliterated this wrongly because I am using an English translation of a dissertation on dhikr originally written in Urdu. Any brothers or sisters who are more knowledgeable should be able to tell us better.)
There is no greater announcement than this. This is the single most important message that Allah wants each and everyone of us to tell every and any person we meet. It means that all else is insignificant in comparision to Allah. Logical thinking and rational reasoning are just mind games. At the bottom of it all, there must be total realization that it is Allah and only Allah that makes the first and the last decision. The ability to think is itself a blessing from Allah. To assume that just because we are able to think, we are able to reason out the truth about Allah is a delusion. It would be kind of like the fish that thought it knew all about the fisherman just by applying logical thinking and rational reasoning to analyse the bait dangling in the water.
Alternatively, as a very rough analogy, consider the ubiquitous mobile phone. Just because we own the very latest model with all the very latest slick gadgets and all the very latest cutting-edge gizmos does not mean that we have in our hand the ultimate communications device. It is the service provider, the base station, that calls the shots. Do we think our mobile phone is of any use if it is not connected to the network? Likewise, do we think that we, as sentient beings with such a highly developed level of intelligence, would be of any use if we refuse to connect to Allah, the one and only source of intelligence?
Hope that is useful. Insha Allah.