これは我々だけの秘密だ。

this (indicating an item near the speaker, the action of the speaker, or the current topic); this person (usu. indicating someone in one's in-group); now; here; I (me); certainly

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

we

only; just; merely; simply; no more than; nothing but; alone; as (e.g. as soon as, as much as); worth

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

secret; secrecy; confidentiality; privacy; secret; mystery; esoteric teachings

be; is; indicates past or completed action; indicates light imperative