A priest wrote that we should humbly accept that our church, whatever our denomination, does not have the whole truth. But I thought the Catholic Church was the pillar and foundation of the truth.

Here, too, the subtleties of language are important in understanding how questions are answered. The Catholic Church maintains, for demonstrable reasons, that we possess the fullness of revealed truth and the full means to salvation given by God. We hold this in distinction to other denominations and religious traditions, which may have elements of the truth, but are lacking the fullness of these and are usually admixed with error. 
 
That said, we do not claim to know everything there is to know about God. God is more glorious than everything we could ever say or know about him. 
 
It is perhaps in this vein that the priest wrote his remarks. Nevertheless, one might wish for greater precision from the good father, as your puzzlement demonstrates.