How can we be entirely honest and yet not speak that which is truth?
I would say because we are bound by our human limitations … bound, constrained, limited. As we are able to expand our boundaries by overcoming our limitations we may honestly change our opinions* and move closer to that which is true.
*Opinions―actions, judgements, conclusions … world view …
When we are honest we speak the truth as we understand the truth to be.
Our understanding is limited. We are not; omniscient, omnipotent, omnipresent. Our understanding is limited to what we individually do know. A humble heart is most conducive to an honest answer …