"It cannot be too firmly realised"?

Solution 1:

Firmly realized means very well understood. Too firmly realized means understood more urgently than you really need. Cannot too firmly realize means that no matter how urgently you understand this it is still not enough. It comes out as slightly awkward English to my ears but YMMV.

He means that it is impossible to over emphasize that fact. With the double negative as in the comment the phrase might be; It cannot be over emphasized that every soul in incarnation is down here for the specific purpose.

The usage of this is more often seen I think as a separate sentence tacked on to the end of the claim. This cannot be over emphasized.

Look at Dickens A Christmas Carol page one for "This must be firmly understood..."