The Greeting 
The Greeting "When you see someone you don't know use the greeting. Everyone who follows Orlanth knows the greeting. If they do not answer rightly at least you know where they stand. When my brother was outlawed he travelled all the way to Pralorela, a year of walking and hunting, and everyone he met greeted him properly." Aski Harbardsson, thane of the Varmandi clan [Glorantha: Genertela, Player's Book p.14]
What exactly do they say? One possibility is from the myth, "The First Hospitality:"
... When [people] came, Umath and his officers greeted them. He said the sacred words, which could not be said if falsehood was intended.
The first visitor was Vestkarthan of the Deep. When he came close to Umath's camp he was challenged.
"Halt, Stranger! Who comes this way, to a place which is not allowed to everyone? Do you come in friendship, or as a foe?"
"Greetings, guardian. I come as a friend."
"Greetings then, Stranger. You speak to the Lord of this place. I am Umath, Champion of the gods and slayer of Howling Void, and liberator of Justice. Tell me your name if you are a friend."
[Umath and Vestkarthan exchange offers of hospitality and promises of praise]
And so Umath welcomed the guest into his camp, and this is still the way that all people have been greeted into our homes and sacred places. [King of Sartar p.61]
This obviously contains elements of the greeting, but seems specialized to inviting a stranger in. It's not as appropriate for challenging someone who wanders onto your pasture lands. Also, the greeting has to work in different languages, so the exact words can't be too important. And it's cross-cultural -- Aski's brother would be a Heortling, but he travelled through lands inhabited by Haranding Orlanthi.
Everyone who is a native speaker of a Theyalan language knows the greeting. A bad analogy: everyone who is a native French speaker knows how to greet someone by kissing cheeks. The greeting is, in my mind, totally unconnected with the cult of Orlanth (an Elmali clan would have essentially the same greeting).
The greeeting is both challenge and hospitality. I think it's mostly hospitality, and the challenge part is just to make sure you don't accidentally give hospitality to a clan enemy you don't know personally.
Hospitality is pretty much a universal custom, though the greeting is a specifically Theyalan (Orlanthi) custom. For example, Praxians have a meeting ceremony which tests a stranger's courage [Tales of the Reaching Moon 14.38], and the Doraddi of Pamaltela have meeting contests.
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