To deepen in the Cause means...

"To deepen in the Cause means to read the writings of Bahá'u'lláh and the Master so thoroughly as to be able to give it to others in its pure form. There are many who have some superficial idea of what the Cause stands for. They, therefore, present it together with all sorts of ideas that are their own. As the Cause is still in its early days we must be most careful lest we fall under this error and injure the Movement we so much adore.

"There is no limit to the study of the Cause. The more we read the writings the more truths we can find in them the more we will see that our previous notions were erroneous."

(From a letter written on behalf of Shoghi Effendi to an individual believer, August 25, 1926)

http://bahai-library.com/hornby_lights_guidance_2.html&chapter=4#n1914