Paul says that God has revealed previously hidden truths to the believers. He speaks of these truths as if they were a secret. You can translate this in an active form. Alternate translation: "because God has revealed to us believers the secret that he was keeping for long ages" (See: [[rc://en/ta/man/jit/figs-metaphor]] and [[rc://en/ta/man/jit/figs-activepassive]])