a 'diagnosis' is the identification of the nature of an illness or other problem by examination of the symptoms.
So, it could be a doctor diagnosing a fatal illness, or a motor mechanic making a diagnosis as to what is causing that 'knocking' noise in your car engine.
A doctor makes a diagnosis and tells me I have cancer and 6 months to live. If I refuse to believe his diagnosis...well...come 6,7, or perhaps 8 months, and I'm dead and the autopsy reveals cancer: the truth of his diagnosis was revealed 'in the future', 6-8 months later when I died of cancer.
If I lived for another 20 years and autopsy showed I died of a heart attack, and no signs of cancer were found in my body, then after one year, never mind 20, it was apparent the diagnosis was wrong.
The truth (I have cancer) or otherwise (he was wrong - I didn't have cancer) was revealed in time.