Epigraphy (1)

                   DIE XVII SEPTEMBRIS MMX
                             BENEDICTVS XVI
   IN BRITANNIAM A REGINA ELIZABETH INVITATVS
MISSAM PRETIOSISSIMI SANGVINIS HIC CELEBRAVIT
      FIDES IN SOCIETATE QVOD COMMODVM FIT
   BEATIFICI J H NEWMAN EXEMPLO ILLVSTRANS

This is what you walk over now as you cross the lintel into Westminster Cathedral. It's all very straightforward, it seems to me, except for line 5. A 'translation' is offered in the November 2013 number of OREMUS, the Westminster Cathedral Magazine, but I wonder if you might care to consider three questions before you look at that. The plain meaning seems to me to be "because keeping ones oaths in an alliance becomes an advantage", but I have a suspicion that this rather military and Livian sentiment isn't what the author thought he was saying! Questions, then:
(1) What did the author want to say?
(2) How would you put into Latin what you think he wanted to say?
(3) Does (1), in any case, fairly summarise the sermon that our Holy Father did preach on that occasion?

In a couple of days, I'll publish my own answer to those questions.

[BEATIFICI is odd. I mean, Blessed John Henry Newman does make us happy ... or me, at least ... but 'happy-making' is a strange epithet to assign to him. I wonder if someone was under the impression that it meant 'beatified'.]