When Jesus appears among the frightened disciples, his first words are 'Peace be unto you.' The phrase is not an invented Christian formula appearing only after the resurrection.
Greetings of peace were embedded in Jewish and wider Semitic patterns of speech and carried associations of wholeness, safety, and well-being.
The Hebrew shalom and related Semitic expressions can refer to peace in a broad sense: welfare, completeness, security, and harmonious relationship. Asking after someone's peace or pronouncing peace upon a person was therefore a natural social greeting.
Luke places this conventional greeting in an extraordinary setting. The disciples are startled and afraid, believing they may be seeing a spirit, while the crucified Jesus stands alive among them.
The cultural background allows the greeting to remain ordinary and profound at the same time. Jesus speaks the familiar language of peace into a room filled with fear and confusion.
Because the speaker is the risen Jesus, the greeting also embodies the restoration the disciples are beginning to experience: the one whose death shattered their hopes now stands among them alive.