known: recognized as fact, or, of a person, someone one is acquainted with
This is the adjective formed from the common verb γινώσκω, "to know." At its root it means simply "known," the state of having been recognized. Everything it does grows from that one idea, and it splits only by what is known: a fact, or a person.
Most often in the New Testament it describes information: a report, a healing, or a truth that has become public. Luke uses it this way again and again in Acts, especially in the fixed phrase that opens a speech, "let it be known to you." Here the natural English word is just "known."
Applied to a person, "known to someone" shades into "an acquaintance," someone you personally know. The relatives and friends searching for the boy Jesus, the circle standing at a distance from the cross, and the disciple who had an entry to the high priest's courtyard are all described with this word. English switches words at that point, from "known" to "acquaintance," though the underlying idea is unchanged.
Nuances
Whether "what is known of God" in Romans 1:19 means the knowledge people actually have of God or the knowledge of God that is available to be had (the knowable).
- contestedIt means what people actually know about God, the knowledge they in fact possess.; Cranfield, C.E.B., A Critical and Exegetical Commentary on the Epistle to the Romans (ICC), on 1:19
- contestedIt means what can be known of God, the knowable, God's knowability.; BDAG, s.v. γνωστός