The Christianity sense (sense 1) was first used by early Christians in the 1st century AD; the Latin cross design represents the cross upon which Jesus was crucified around AD 33 (see also cruxifiction of Jesus on WikipediaWikipedia ). It was popularised as a universal symbol for Christianity after circa AD 350, overtaking symbols like the ichthys.
The death senses (sense 2) might be derived from the cross as a method of execution in the form of crucifixion, while the grave subsense (sense 2.1) is probably derived from the common use of a Latin cross as a grave marker to indicate that the buried individual adhered to Christianity: reinterpreted as a generic symbol of a grave.
Alternative form of T(“twentieth letter of the English alphabet”), used in some fonts or (Internetslang, sometimes humorous) as an emoji, stylistically.