truly, really: marking that something genuinely matches the truth or the facts
This is the adverb built on the Greek word for what is "true." It stamps something as actually so: not a guess, a rumor, or a hopeful claim, but the real state of affairs. English "truly" or "really" carries it cleanly.
Most often it certifies a recognition as correct. When the soldiers at the cross say a man was "truly the Son of God," or the Samaritans say Jesus is "truly the Savior of the world," the word insists the confession lines up with reality. A close cousin, ὄντως, makes almost the same move but leans on real existence ("really, actually"); this word leans on correspondence to the truth.
A second, sharper use marks something as the genuine article rather than a nominal one. Jesus calls Nathanael "truly an Israelite," and tells those who hold to his word that they are "truly" his disciples: the real kind, not the in-name-only kind.
In Luke it also serves as a solemn lead-in, "truly I tell you," the same weight that the borrowed Hebrew affirmation ἀμήν carries elsewhere in the Gospels.