What if a very senior monk disrobes, how can we protect our mind from losing faith in such situations?

Ajahn Dhammasiha points out that our refuge in terms of Sangha is:

"...Yadidaṃ cattāri purisayugani, aṭṭha purisapuggala - Esa Bhagavato Sāvakasangho...

"...Namely the four pairs of persons, the eight kind of individuals - THESE are the Sangha of the Exalted One's disciples..."

In the strict definition given by the Buddha himself, 'Sangha' as refuge is NOT the ordained sangha of monks and nuns, but it is only those who have realized one of the stages of awakening, whether lay or ordained.

If we keep that in mind, and direct our faith not to just to individual monks or nuns, but to the community of those who have realized the Dhamma, our faith will not be shaken if an individual monk or nun disrobes or acts in uninspiring ways.

Naturally, as we can't know who truly has realized stages of awakening, there is a tendency to regard the ordained sangha as a symbol for the community of awakened disciples, as we can actually recognize monks and nuns easily by their wearing robes, and our faith likes to have something visible and concrete to latch on. There is nothing wrong in that, especially as so many Arahants and noble disciples have been part of the ordained sangha.

But even so, we should be mindful to direct our faith to the WHOLE sangha, the complete sangha of the four quarters and the three time periods, rather than only to individual monks or nuns who we feel personal affection for. Again, this will protect our faith from crumbling in case a monk or nun we like does not meet our expectations.