Not only does Trinitarian theology shape the goal toward which the Christian mission is striving; it also clarifies the means which are to be used in the pursuit of that goal. Redemption is ultimately an accomplishment of the triune God; he alone is the doer of the work, and therefore, any human activity must flow from his prior activity, and be directed and empowered by him. The mission that God left his people with is ultimately his mission, and advances on the basis of his eternal, immutable design; and so, any human activity which fails to take into account God's redemptive plan as he has made it known is bound to be frustrated. Human mission endeavors are likely to be successful only as they understand the divine agenda and lean upon divine strength. This means that a first qualification for any missionary is a knowledge of the triune God; an awareness of the role of the persons of the Godhead in the work of redemption, as revealed in the scriptures; and a heart-attitude of faith in those joint operations of the persons of the Trinity.
For example, take the scriptural revelation of the work of the Father in the plan of redemption: he is the ultimate planner, the source from whom the whole work flows and is governed. We see throughout the gospel of John that the Son, in the fulfillment of his part of the redemptive work, acts in an unceasing obedience to the Father's will (e.g. John 5:17-19, 30; 8:28-29; 10:17-18; 14:31; 17:4). Likewise the Spirit, when he comes, speaks not on his own, but only what he has heard from the Father and the Son (John 14:26; 15:26; 16:13-14). This role of the Father in planning out the work of redemption is seen with special clarity in the aspect of his choosing its subjects. We have already observed that the Father has chosen a specific people to give to the Son, and that the Son has purposed to redeem these alone (e.g. John 6:37-40; 10:29; 17:1-2, 6, 10); we may add to this testimony the witness of the epistles, which speaks of the Father's choice of a certain people to be redeemed in no uncertain terms (e.g. Romans 8:29-30; Ephesians 1:3-6; 1 Peter 1:1-2). We may learn further from the revelation of scripture that this people is chosen out of every kindred, tribe, tongue, and nation (e.g. Revelation 5:9), and that it will be called out only when the gospel is proclaimed in all the world (e.g. Matthew 24:14).
So how does this truth affect the task of the Christian missionary? First, it gives him a clear directive in the pursuit of the task: as the Church continues to spread across the world, believers may know that in their missionary endeavors they ought to target the kindreds, tribes, tongues, and nations which are yet unreached, because they know that the conversion of representatives from these peoples is the Father's will. Their task remains undone as long as there is any people group that has not heard the gospel, or that has not yet seen fruit from the proclamation of the gospel. Second, this understanding gives hope to missionaries laboring in the most difficult places. When Paul was experiencing opposition in Corinth, he was comforted by the realization that the Father had many people in that city, chosen for a redemption which had not yet been applied (see Acts 18:9-11). In the same way, the missionary who understands the biblical representation of the Father's role in redemption has a strong hope that his labor will not be in vain, and has cause to cry out to God in faith for the success which has been promised. Because God has chosen a people, our ultimate success is guaranteed. This foundational awareness of the Father's revealed role in the work of redemption drives a faithfulness which would otherwise wilt under the discouragement of unfavorable circumstances.
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-- from How the Doctrine of the Trinity Shapes the Christian Mission