Soldiers in God’s army?

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:

Phil. 2:25 But I thought it necessary to send to you Epaphroditus, my brother and fellow worker and fellow soldier, who is also your messenger and minister to my need;
Phil. 2:26 because he was longing for you all and was distressed because you had heard that he was sick.

Philem. 1 Paul, a prisoner of Christ Jesus, and Timothy our brother, To Philemon our beloved brother and fellow worker,
Philem. 2 and to Apphia our sister, and to Archippus our fellow soldier, and to the church in your house:

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:

Is. 63:1       Who is this who comes from Edom, 
    With garments of glowing colors from Bozrah, 
    This One who is majestic in His apparel, 
    Marching in the greatness of His strength? 
    “It is I who speak in righteousness, mighty to save.”
Is. 63:2      Why is Your apparel red, 
    And Your garments like the one who treads in the wine press?
Is. 63:3      “I have trodden the wine trough alone, 
    And from the peoples there was no man with Me. 
    I also trod them in My anger 
    And trampled them in My wrath; 
    And their lifeblood is sprinkled on My garments, 
    And I stained all My raiment.
Is. 63:4      “For the day of vengeance was in My heart, 
    And My year of redemption has come.
Is. 63:5      “I looked, and there was no one to help, 
    And I was astonished and there was no one to uphold; 
    So My own arm brought salvation to Me, 
    And My wrath upheld Me.
Is. 63:6      “I trod down the peoples in My anger 
    And made them drunk in My wrath, 
    And I poured out their lifeblood on the earth.”

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. 

Scripture quotations taken from the (NASB®) New American Standard Bible®, Copyright © 1995, 2020 by The Lockman Foundation. Used by permission. All rights reserved. www.lockman.org