Many Christians will probably be familiar with this symbol:
This is the so-called staurogram, composed by the Greek letter tau (Τ) superimposed on the letter rho (Ρ). As the name implies, this is a symbol of the cross (stauros in Greek). While the device is actually pre-Christian (the symbol was originally used as an abbreviation for “three” or “thirty” - treis and trianta, respectively) the symbol was used by early Christians to abbreviate the Greek word stauros (“cross”) or stauroō (“crucify”): it is used in this way in very early New Testament manuscripts such as Papyrus 66 (ca. early 3rd century), Papyrus 75 (ca. AD 175-225) and Papyrus 45 (ca. mid-3rd century), almost like a nomina sacra.
(What is a nomina sacra, BTW? Nomina sacra (Latin for ‘sacred names’) is a Christian scribal practice of abbreviating certain words or names which have some connection with God or Jesus - “Lord,” “God,” “Jesus Christ” - and thus are deemed to be in some way sacred. These contractions were usually indicated with overlines and are formed by taking the first one or two letters of the words, plus the final letter(s) to determine the inflection - since Greek is an inflected language, meaning that each word actually changes form, or inflection, based upon the role that it plays in the sentence. See this page for examples.)
A staurogram in Papyrus 75: Chttps://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Christliche_Symbolik_%28Menzel%29_I_193_2.jpg/20px-Christliche_Symbolik_%28Menzel%29_I_193_2.jpgON (st[au]ron, Luke 14:27)
The tau-rho is actually distinct from other monograms related to Christ (Christograms) in that the letters do not really derive from any letter contained in Jesus’ name (IHCOYC XPICTOC, Iēsous Christos) or in titles given to Him. The more popular chi-rho (https://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Christliche_Symbolik_(Menzel)_I_193_1.jpg/20px-Christliche_Symbolik_(Menzel)_I_193_1.jpg) is composed by the first two letters of Christos (XP); the iota-chi monogram (https://upload.wikimedia.org/wikipedia/commons/thumb/9/99/Christliche_Symbolik_(Menzel)_I_193_4.jpg/20px-Christliche_Symbolik_(Menzel)_I_193_4.jpg) is formed by taking the first letters of Iēsous (I) and Christos (X), respectively. Moreover, the staurogram is the only one of these monograms that appears as part of nomina sacra, which would mean that it functioned differently as an early Christian symbol.
Now I think some of you will be familiar with the idea that Christians did not visually portray the crucified Jesus until the 4th-5th century, with some even drawing conclusions from this apparent absence. (The so-called Alexamenos graffito doesn’t count as it was made by a non-Christian who is mocking Christians.) A scholar named Larry Hurtado (his blog) disagrees with this idea; he instead argues that the staurogram could actually be the earliest extant Christian visual reference we have to the crucified Christ (the T = the cross; the loop on the P = the head of the crucified). The staurogram, in his opinion, were not simply abbreviations to save space in manuscript but were visual expressions of Christian piety.
See for example his book The Earliest Christian Artifacts: Manuscripts and Christian Origins, pp. 135-54, and the article The Staurogram in Early Christian Manuscripts: The Earliest Visual Reference to the Crucified Jesus? (alternate link), as well as some blog posts by him.