spoken the whole truth to you

"spoken honestly to you"

our heart is wide open

Paul speaks of his great affection for the Corinthians as having a heart that is open. Here "heart" is a metonym for a person's emotions. Alternate translation: "we love you very much" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-metonymy)