Abbreviations and Symbols
The letters IHS are a Latinized form of the first three letters of the Greek version of the name “Jesus”. The full Greek form would normally be: IHΣOYΣ but the letter Σ (sigma) is usually converted to an S, and indeed in Gothic script it often looks like a letter C. (Note: Despite popular belief, the three letters are not the abbreviation of some Latin phrase.)
Similarly XP (the X is often written on the stem of the P) are the first two letters of the Greek word XPIΣTOΣ, usually transliterated as “Christos” or “Christ”. The symbol is usually known as the “Chi-Rho” because Chi and Rho are the names of the two letters. (They do not correspond to the two English letters they resemble.)
The letters AMDG occur on quite a lot of the items in some churches. The abbreviation stands for the Latin phrase:
Ad Maiorem Dei Gloriam
To the Greater Glory of God
These letters most often occur at the top of a crucifix, but are sometimes used to link something else (such as eucharistic symbols) to the crucifixion. They are an abbreviation of:
Iesus Nazarenis Rex Iudaeorum
Jesus of Nazareth, King of the Jews
which is the Latin version of the inscription attached to the top of the cross, originally in Latin, Greek and Hebrew.
The fish is probably the oldest symbol used by Christians to represent their faith, predating the use of the cross by several hundred years. Besides being appropriate because of the occurrence of fish and fishermen in the story of Jesus, it is also an acrostic. It is based on the expression:
IHΣOYΣ XPIΣTOΣ ΘEOY YIOΣ ΣΩTHP
Jesus Christ, Son of God, Saviour
The first letters of each word of the Greek expression themselves make up the word IXΘYΣ, (or ichthus) which is the Greek word for “fish”.
The traditional symbols of the four evangelists (ie the writers of the four Gospels). Matthew is represented by a man, Mark by a lion, Luke by an ox, and John by an eagle. In practice all four are often supplied with wings.
The set of four comes originally from the Old Testament book of Ezekiel. (See Ezekiel 1.4-14, especially v10 - the description there is of the Cherubim, the “living creatures” who were the throne-bearers of the Almighty.) The picture is taken up again in the book of Revelation (see especially Revelation 4.7).
The idea of the symbolism is that the Gospels present four faces of one truth. The meaning of each is that Matthew is supposed to be the Gospel which emphasises the human side of Christ, Mark the kingly side, Luke the side of Christ as a sacrifice, and John gives an over-arching vision. (Modern theologians might well dispute these understandings, but this is the traditional view!)
Alpha and Omega, the first and last letters of the Greek alphabet. Together they mean “the beginning and the end”, so between them symbolize eternity. The expression is used explicitly of God in Revelation 1.8.
It used to be thought that the female pelican fed her young with her own blood. This can easily be understood as a symbol of Christ giving his own blood for mankind. The depicition of the pelican feeding her young in this way is often referred to as the "pelican in her piety".