。
It was pure chance that we met.
Literal
We [subject-が] met [nominalizer-の] [topic-は] completely [of-の] coincidence was.
Cleft-style focus: [僕たちが出会ったの] は [まったくの偶然] でした — 'the thing that we met was pure coincidence.' の nominalizes the relative clause, は topicalizes it as the subject of identification. まったくの偶然 = '[a matter] completely [of] coincidence' — まったく + の creates an attributive ('utter, complete').