Since most people could not read, the use of symbols was very important to communicate the teachings and identity of the early Christian church. Some of these symbols were Christian inventions; some were influenced by symbolism from other religions. Here are some objects showing early Christian imagery from around the Mediterranean world and one from England.
Coin of Emperor Constantine

Coin of Emperor Constantine AD 306 – 337 from Croatia, showing the emperor praying.
See more See more: https://www.google.com/culturalinstitute/beta/asset/gold-medallion-showing-constantine-the-great-at-prayer/3AEVCVZ6bG5TfQNecklace from Istanbul

Part of a necklace from Istanbul dating to the AD 300s with crosses and a chi-rho symbol.
See more See more: http://www.britishmuseum.org/explore/highlights/highlight_objects/pe_mla/e/element_of_a_necklace.aspxDepiction of the crucifixion

Ivory plaque from Rome with the earliest known depiction of the crucifixion, AD 420 – 30.
See more See more: http://www.britishmuseum.org/explore/highlights/highlight_objects/pe_mla/p/panel_from_an_ivory_casket_th.aspxStone frieze with an eagle or dove and a Coptic cross

Part of a stone frieze with an eagle or dove and a Coptic cross from Sudan, AD 600s.
See more See more: http://www.britishmuseum.org/explore/highlights/highlight_objects/aes/s/sandstone_frieze.aspxPainting showing saints

Wall painting showing saints with haloes from Egypt, AD 500s.
See more See more: http://www.britishmuseum.org/explore/highlights/highlight_objects/aes/w/wall_painting_of_the_martyrdom.aspxBuckle with Christian symbol of a fish

An Anglo-Saxon buckle from Kent with the Christian symbol of a fish; mid AD 600s.
See more See more: http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?objectId=95555&partId=1&searchText=buckle+fish&page=1