I’m currently working through the text and vocabulary of Second Timothy, writing Lexical Commentary on the Pastoral Epistles: Second Timothy. I ran into the curious phrase in 2Ti 2:3, “as a good soldier of Christ Jesus.”
This actual phrase “soldier of Christ” only occurs here in the NT. This got me digging a bit because — having grown up in the church — the notion of “Christian soldiers” is not unfamiliar. I was surprised that the only other relatively early witness to this phrasing was found in the Acts of Paul. Of course there is other imagery (e.g. “Christian Armor” in Eph 6:10–20), but I was looking at the expression of a Christian as a soldier.
It was only when I expanded my search from the specific phrase to words sharing common roots that I remembered the term “fellow solider,” which Paul uses twice elsewhere. If you’re looking at word usage in a corpus, sometimes you have to expand your basis because the same notion/concept can be expressed using different formulations.
Here’s what I ended up with, at least for now. This will change some between now and whenever I get the manuscript done, but I’m over 25% of the way through the text, which is good news for me.
The phrase “good soldier” is a translation of two different Greek words. The first word, “good,” is a translation of καλός. The second word, “soldier,” is a translation of στρατιώτης, ‘soldier,’ in a non-military sense. The Acts of Paul uses the term similarly:
The word στρατιώτης is used with frequency in the New Testament. While military language is used elsewhere in descriptions of the Christian life (Eph 6:10–20), the direct association of a soldier (στρατιώτης) with a follower of Jesus is only found in 2 Timothy. The next known usage of this phrase “soldier of Christ” is found in the Acts of Paul which may go back to the second century.
However, the word συστρατιώτης, ‘fellow soldier,’ is used to similar effect in Phm 2 and Php 2:25:
Paul, a prisoner of Christ Jesus, and Timothy our brother, to Philemon, our dear friend and fellow worker, and to Apphia our sister, and to Archippus our fellow soldier, and to the church in your house. (Phm 1–2, emphasis added)
But I considered it necessary to send to you Epaphroditus, my brother and fellow worker and fellow soldier, but your messenger and servant of my need, because he was longing for all of you and was distressed because you had heard that he was sick. (Php 2:25–26, emphasis added)
In these contexts, Paul uses συστρατιώτης to associate Archippus (Phm 2) and Epaphroditus (Php 2:25) with him in common mission for the gospel. They are described as soldiers in common with Paul. They hold common purpose and share a common battle. Understanding Christians as soldiers in common battle is not outside of the frame of Paul or the New Testament.
 BDAG pp. 504–505. Occurs 101x in NT, 24x in PE.
 BDAG p. 948. Occurs 26x in NT, 1x in PE: 2 Ti 2:3. See also TDNT 7.711–712.
 Wilhelm Schneemelcher, “3. The Acts of Paul,” in New Testament Apocrypha, ed. Wilhelm Schneemelcher, trans. R. McL Wilson, vol. 2, 2 vols. ([Cambridge, England]; Louisville, Ky.: J. Clarke & Co. ; Westminster/John Knox Press, 2003), 259.
 From the Hamburg papyrus of the Acts of Paul, which BDAG cites as AcPl Ha 8, 9
 Hans-Josef Klauck, The Apocryphal Acts of the Apostles: An Introduction (Waco, TX: Baylor University Press, 2008), 48.