In Chapter Four of this novel we are presented with the character of Brocklehurst, and the overwhelming impression we receive of him, and in particular his "brand" of Christianity, is that he is a massive hypocrite. Of course, Jane as a character who feels she must speak the truth and stand up against injustice and hypocrisy does not help herself in her conversation with Brocklehurst. You will want to pay attention to how Brocklehurst is introduced by the first person narration - we first see him from Jane's point of view and he is described as follows:
...I looked up at - a black pillar! - such, at least, appeared to me, at first sight, the straight, narrow, sable-clad shape standing erect on the rug: the grim face at the top was like a carved mask, placed above the shaft by way of capital.
The reference to Brocklehurst as a "black pillar" indicates the harsh, unyielding nature of his Christianity and character, and the face being described as a "carved mask" likewise reinforces impressions of his hypocrisy. This is emphasised by the allusion to "Little Red Riding Hood", when Jane comments:
What a face he had, now that it was almost on a level with mine! what a great nose! and what a mouth! and what large prominent teeth!
This echo of this famous fairy tale compares Brocklehurst to the wolf who is dressed up as the grandmother, thus making us doubt his words and seeing him for the dangerous character he really is.
Thus, before we come to the part you have highlighted, we already have ample evidence to condemn and despise Brocklehurst. However, let us look at the anecdote. In response to Jane's dislike of the Psalms, Brocklehurst tells her this story:
"I have a boy, younger than you, who knows six Psalms by heart: and when you ask him which he would rather have, a ginger[bread nut to eat, or a verse of a Psalm to learn, he says: 'Oh! the verse of a Psalm! angels sing Psalms;' says he, ' I wish to be a little anger here below;' he then gets two nuts in recompense for his infant piety."
First this shows the manipulation Brocklehurst uses to promote "infant piety" in his charges, but it also presents an overly naive child who supposedly prizes piety over food. As we discover when Jane moves to Lowood, this is something that is core to Brocklehurst's beliefs about his charges spiritual education. Food is not a necessity according to him, and would only spoil the girls. Starvation, apparently, is good for the soul.
This story them continues to develop the contempt that both Jane and we as readers feel for Brocklehurst by presenting Brocklehurst's hypocrisy and unthinking approach to Christianity.