Why does Dante tell what appears to be the self-indulgent tale about smashing the baptismal font in Florence?

                                            They seemed to me

Not any wider or smaller than those designed

for the baptizings in my fair San Giovanni-

One of which many years ago I broke,
To save one drowning there:
(lines 14-18) 

Dante describes the 3rd ditch of circle 8 to contain multiple holes the same size of the holes which lie next to the baptismal tanks in the Florentine churches. It is important to note Dante's word choice as he describes these holes.  When Dante talks of these hellish holes, he does not use words such as 'similar to' or 'approximately'; rather he stresses their direct correlation to the Church's holes: "Not any wider or smaller than those designed". They are identical. He is highlighting the exactness of this comparison and saying 'watch reader, watch for this is specific and "not wider nor smaller', but it is exact. Through this line, Dante is saying in bold letters 'I am about to make a direct statement which contains exact significance. It is important to note that every line of Dante's Inferno has purpose. He is highlightind the fact that this line has purpose here by his word choice.

In this Canto, Dante makes some very bold and at worst blasphemous statements about the Church. He casts three popes in Hell, and he declares their very presence has produced fissures in the rocks. In Matthew 16, Jesus declares that "on this rock I shall build my church". With that scripture in mind, Dante is presenting the case that the popes have cracked and broken the rock of the original church. Dante is doing exactly what he did at San Giovanni, he is damaging the elements of the church. However since the reader has already been told by Dante that the above passage is exact and therefore purposeful, then we must take into serius consideration what happened in those above lines. He destroyed parts of the church to save someone. Dante is then also saying that he is breaking the church "to save" it from "drowning". Dante is exposing the dire faults of the church only to save it.