This study focuses on saddhāyikā in DN23 with its annotations. In particular, I’m interested in the idea that people believe something because they tend to believe people they know rather than experts.
To get there, let’s run through the basic forms of the root word. As a pāli beginner and lay practitioner, I welcome all corrections and improvements 
- saddhā the noun form = faith, belief, or trust (in someone or something)
- saddahati the verb form = have faith (in), believe (in), have confidence (in)
- saddhāyikā the adjective form = (someone or something) in whom one has faith or confidence
For Pali nerds, saddhāyiko is also the adjective form but in the nominative. Saddhāyikā is the adjective form in the instrumental declension. I think i got that right, anyway.
Fun aside:
This word family derives from the Sanskrit root √dhā, which literally means to position or arrange (something). With that nugget, you might intuit how we eventually get to saddhā, for example, in pāli. Or how we end up with paṇidhi in the lovely compound expression in Kp5:
attasammāpaṇidhi = “being rightly resolved in oneself”
On (1) above, we recognize it as one of the five faculties. This brief article does a good job explaining the various permutations of its dhamma meaning.
What we won’t find in the article is any mention of “beliefs” or believing in something or someone. Of course not. The Buddha espouses a highly empirical process for understanding the nature of things. We see for ourselves. We cultivate faith that way.
On (2) above, I haven’t done any research on its use in the suttas.
On (3) above, I show saddhāyikā in DN23 (“With Pāyāsi”):
Bhavanto kho pana me saddhāyikā paccayikā, yaṁ bhavantehi diṭṭhaṁ, yathā sāmaṁ diṭṭhaṁ evametaṁ bhavissatī’ti.
“I trust you and believe you. Anything you see will be just as if I’ve seen it for myself.”
We see our focus word saddhāyikā meaning “I trust you.”
(I’m ignoring the exact grammar here.)
What’s the context? At the beginning of the sutta we learn this:
At one time Venerable Kassapa the Prince was wandering in the land of the Kosalans together with a large Saṅgha of five hundred mendicants when he arrived at a Kosalan citadel named Setavyā…Now at that time the chieftain Pāyāsi was living in Setavyā.
Through a series of similes Kassapa tries to convince Pāyāsi that his strong materialist views are, in fact, wrong view. Early on in the debate, Pāyāsi challenges Kassapa – my very rough narrative, not a translation 
:
Imagine my closest friends and family have kept the five precepts. They all get sick. I go to them on their deathbed telling them “The Buddhist experts say you should all be reborn in a good place. Be of good cheer. But please come back to me after you’ve been reborn and tell me that’s happened. Because the only way I’ll believe it’s true is that you told me so and I trust you.”
Toward the end of the sutta, Pāyāsi takes refuge in the Buddha. Then a few more interesting twists and turns. It’s really delightful to read this sutta. Almost with Shakespearean comedy. As Bhante Sujato notes,
Say what you will about Pāyāsi, he had character.
Actually, an important annotation for me is this, referring back to Pāyāsi’s need to hear back from people he knows and trusts:
This is still a major factor in shaping belief. People will reject the opinions of experts and believe people that they know.
I don’t detect Bhante’s saying this is right or wrong; it’s just how it is.
A Buddhist might say, I don’t really aspire to beliefs because the path isn’t based on that. It’s based on my direct experience of how things are, through the practice, the gradual training.
The challenge is that most everyone else is shaping their beliefs about how things are based on their personal relationships. I mean, what did we just see in the US presidential election cycle. The candidate who won has an uncanny ability to make a lot of people feel like he knows them personally – and like they know him personally. For about 52% of the US population it didn’t matter what factual information was out there.
We owe it to one another to be in civil discourse about what matters in life. For most people, beliefs influence how they assign meaning to life. In this way beliefs are closely related to our values. We assess the worthwhileness of life through the lens of what we believe matters.
I would argue this ends up being a practice, too. In fact, without the occasional clarion call to attend to what matters in life, we naturally lose energy and vitality.
Wouldn’t Kassapa count as an expert whom Payasi didn’t know personally (until now) and thus doesn’t rise to the standard of someone he knows and trusts?
and look forward to reading DN23 from cover to cover – many times!
when he realized Āḷāra Kālāma couldn’t be his teacher.)