A way to enable data encryption by default (device to be encrypted on first boot)
Concerning the asking of password. When resetting Marshmallow, if the stock recovery is in place, it decrypts the phone and on first boot it encrypts it but well obviously never ask you for a password to start android because these two fine are recreated which hold device password:

/data/security/password.key
/data/security/pattern.key


If you could have been using TWRP to factory reset and you had a pattern or device assigned to device and you've opted for require password to start device, seamlessly, on any ROM and even TWRP itself. The password will exist even after resetting. The stock recovery is the only one that deals with that pattern at first boot.