Everyone is familiar with the gospel narrative of Christ entering triumphantly into Jerusalem on Palm Sunday. Less familiar, perhaps, is why the palm was used and why it was later adopted in Christian usage. The article below explains.
Palm in Christian Symbolism
In pre-Christian times the palm was regarded as a symbol of victory (Aulus Gellius, “Noct. Att.”, III, vi). It was adopted by the early Christians, and became a symbol of the victory of the faithful over the enemies of the soul. The palm, says Origen (Commentary on John X.18), is the symbol of victory in that war waged by the spirit against the flesh. In this sense it was especially applicable to martyrs, the victors par excellence over the spiritual foes of mankind; hence the frequent occurrence in the Acts of the martyrs of such expressions as “he received the palm of martyrdom.” On 10 April 1688 it was decided by the Congregation of Rites that the palm when found depicted on catacomb tombs was to be regarded as a proof that a martyr had been interred there. Subsequently this opinion was acknowledged by Mabillon, Muratori, Benedict XIV and others to be untenable; further investigation showed that the palm was represented not only on tombs of the post-persecution era, but even on pagan tombs. The general significance of the palm on early Christian monuments is slightly modified according to its association with other symbols (e.g., with the monogram of Christ, the Fish, the Good Shepherd). On some later monuments the palm was represented merely as an ornament separating two scenes.
“Palm in Christian Symbolism.” The Catholic Encyclopedia. Vol. 11. New York: Robert Appleton Company, 1911. 12 Apr. 2019 <http://www.newadvent.org/cathen/11432a.htm>.