It wasn't until the next morning that we learned the truth.
Literal
Next-morning [becoming-になって] first-time we [topic-は] that fact [object-を] knew.
The ~になって初めて 'not until' frame paired with 知る ('to know / learn of') foregrounds when knowledge finally arrived, and implies that the truth had been hidden up to that point. 我々 is the formal/literary first-person plural pronoun. A classic dramatic-reveal construction in Japanese narrative prose.