英語口語：秘密用英語如何說？

secret; clandestine; confidential; arcanum;
例句：
1.我的秘密武器一直是我的家庭。
My secret weapon was my family always.
2.許多基督徒是在家里秘密集會的。
Many christians were meeting secretly in homes.
3.facebook和其他社交網絡已有數百萬易受傷害的、秘密的低齡用戶。
Facebook and other social networks already have millions of vulnerable, clandestine underage users.
4.此次暗殺中，27個秘密特工使用偽造的護照出行。
The killing involved 27 undercover agents travelling on forged passports.
5.正因如此，一些秘密將繼續保持下去。
So we are left with something of a mystery.