What is the fundamental virtue to be cultivated in order to prepare for the spiritual life?

I have said this many times, but this is an opportunity to repeat it it is sincerity. A sincerity that must become total and absolute, for sincerity alone is your protection on the spiritual path. If you are not sincere, at the very next step you are sure to fall and break your head. All kinds of forces, wills, influences, entities are there, on the look-out for the least little rift in this sincerity and they immediately rush in through that rift and begin to throw you into confusion. Therefore, before doing anything, beginning anything, trying anything, be sure first of all that you are not only as sincere as you can be, but have the intention of becoming still more so. For that is your only protection.

(From the Mother’s answer to a devotees question)