In the twentieth century, the influential German commentary of Martin Dibelius (revised by Hans Conzelmann), Die Pastoralbriefe (4th ed.; HNT 13; Tübingen: Mohr Siebeck, 1966) was mediated to the English-speaking world in the Hermeneia series as The Pastoral Epistles (trans. Philip Buttolph and Adela Yarbro; Philadelphia: Fortress, 1972). One of the key points of influence was the christliche Bürgerlichkeit proposal popularized in the commentary. This idea of the “good Christian citizen” traded on the notion that the Pastorals were written in light of decreased expectation of the parousia, and that in order to survive a hostile world, believers were going to have to learn to settle in for the long haul. In Dibelius’s reading of the Pastorals, “settling in” meant “fitting in,” and the letters were concerned to help Christians maintain a low profile, so to speak, by living in such a way that the surrounding culture would look on with at least a measure of approval. Dibelius’s proposal was heavily grounded in 1 Tim 2:1-2, and found support in the concern with the perception of outsiders found throughout the letters.
The christliche Bürgerlichkeit proposal received significant pushback, however, when the mission-oriented nature of the letters was given its due. The monograph of Philip Towner, The Goal of Our Instruction: The Structure of Theology and Ethics in the Pastoral Epistles (JSNTSS 34; Sheffield, UK: Sheffield Academic, 1989; repr., Bloomsbury Academic Collections; New York: Bloomsbury Academic, 2015), provided an important response to Dibelius, which was later mediated through his influential NICNT commentary, The Letters to Timothy and Titus (Grand Rapids: Eerdmans, 2006).
Towner, however, is not the only one who has emphasized the mission-oriented nature of the letters, and two works in the same vein have recently come to be available.
Chiao Ek Ho’s Aberdeen dissertation, “Do the Work of an Evangelist: The Missionary Outlook of the Pastoral Epistles” (2000), written under I. Howard Marshall, was unfortunately never published as a monograph, though its core substance was made available in “Mission in the Pastoral Epistles,” in Entrusted with the Gospel: Paul’s Theology in the Pastoral Epistles (ed. Andreas J. Köstenberger and Terry L. Wilder; Nashville: B&H: 2010), 241‒267. The dissertation itself may now be accessed at EThOS (here is the link, along with an abstract), and to my understanding has only recently been available. All students of the Pastorals should obtain it.
Additionally, Andreas J. Köstenberger has just produced an article-length treatment: “An Investigation of the Mission Motif in the Letters to Timothy and Titus with Implications for Pauline Authorship.” BBR 29.1 (2019): 49–64 (abstract and full article available here). This article is grounded in (and goes beyond) the biblical-theological work done on mission in the Pastorals as set forth in Köstenberger’s recent Commentary on 1‒2 Timothy & Titus (BTCP; Nashville, TN: Holman, 2017).