A cross is a common symbol used by Christians to indicate themselves as believers in Christ. It symbolizes the very thing used by God to redeem mankind from sin, by putting His Son Jesus Christ to death on a cross (a rather torturous form of death during the time of the Roman Empire) as sufficient payment which Psalm 22 and Isaiah chapter 53 prophesied would happen.
In Soul Harvest, the Seal of God that appears on believers as well as on the 144,000 Jews was a cross mark. In Glorious Appearing, the "sign of the Son of Man" that appeared in the sky prior to Jesus' Second Coming was a cross made of electrical energy. The appearance of the cross was able to heal believers of their injuries and infirmities and make them impervious to the assaults of the Global Community. The authors of the Left Behind series interpret the mention of the "sign of the Son of Man" in the Olivet Discourse (Matthew 24:30) to be fulfilled just before the Glorious Appearing.
In other religious groups
NOTE: This section contains out-of-universe material that is not part of the Left Behind story franchise.
There is some debate whether the cross was the instrument of Jesus' crucifixion. The word used in Greek is stauros (σταυρός). The Jehovah's Witnesses claim that Jesus did not die on a cross, but rather a "torture stake", an upright pole. The Jehovah's Witnesses, currently, equate the cross with a pagan symbol, as they think the cross illustrates the corruption of "true worship", although they did incorporate the cross as a symbol before 1931 (as they were simply known as the Bible Students).