Is there anything regarding God’s chosen soldiers for His army? Is this a group called to fight during the 7 year Tribulation, the Rapture, or when Christ comes to defeat the antichrist?
The New Testament uses the word "soldier" to refer to all Christians in a general sense, not in the sense of an actual military unit. For example, take note of the following verses:
As the passages above demonstrate, Paul uses the term "soldier" simply to refer to every Christian, and he does not use it in the literal sense of combat. Christians are never called to engage in literal warfare for Christ. We engage in spiritual warfare through prayer, evangelism and other disciplines of the faith.
Regarding the Tribulation, Christians do not engage in any battle at any time. Christians are martyred for their faith, and therefore they will not fight back against the Antichrist. When Christ returns to defeat the Antichrist, He alone fights the battle:
This passage describes Jesus arriving at the end of Tribulation to defeat the Anitchrist. Notice in v.5 Jesus said He fought this battle alone with no one to assist Him, for He does not need assistance.
So the term "soldier" in the New Testament is euphemism for every Christian, and therefore it does not refer to a literal army or physical battle. It refers to our spiritual warfare against the enemy.