How does Trinitarian Theology speak about evil?
1. Because God will not coexist with evil, and because there are so many evil people in the world, it follows that God has NOT already adopted all humans.
2. If God has actually adopted all humans already, we should see in the world a corresponding and significant abating of evil since the time God first accomplished this adoption. However, we don't see such abatement having happened - evil is clearly still present with us.
First let me say that Trinitarian Theology does not bury its head in the sand concerning evil. One need only note the courageous response from Karl Barth, Dietrich Bonhoeffer and other Trinitarian theologians to the evil of Hitler and the Nazis. It is clear that there remains in our world a great deal of evil. In fact, one might argue that the ravage of evil upon our world has increased since Jesus' advent (an increase due, at least in part, to "advances" in the technology of warfare).
But that being said, I think the above-mentioned two arguments are "barking up the wrong tree." The adoption of all humanity in Jesus is neither confirmed nor disconfirmed through physical observation of conditions in our world. Rather, this universal adoption is seen clearly only by and in the revelation of the person and work of Jesus Christ. It is a matter of revelation, not observation.
This universal adoption constitutes the in-breaking of the Kingdom of God in our world. And Jesus likens this Kingdom presence to the "yeast," which though unseen, nevertheless infects the whole "lump of dough" (see Mat 13.33). The full revealing (to all eyes) of this inclusion awaits the parousia (revealing) of Jesus and of all humanity included in Jesus. This will constitute the final judgment (krisis), which comes with the general resurrection.
So how do we know of humankind's present inclusion in God through Christ? Because it has been revealed to us in the revelation of Jesus Christ himself. He is shown in Holy Scripture and in the formative Creeds of the Patristic Church to be, in himself, the unique and final union of God and all humanity.
"OK," you say, "but you still have to account for evil in the world!" And, indeed, the gospel does. It says that all humanity is included objectively and universally in Christ: no exceptions; no exclusions. Furthermore, it says that God is NOT separate from sinners; rather he has included all of them; he has adopted the whole lot of us as his children in Christ. Argument #1 above is not correct in its assertion - God is NOT unable or unwilling to coexist with sin; in fact, in Jesus he became sin for us (2 Cor. 5:21). Jesus was (and still is) a "friend of sinners" (see Luke 7:34, and note that God the Father is not of a different mind).
Yes, God hates sin and thus he hates evil because it hurts his children. But he is not afraid of evil, nor is he put off by sin or by sinners. Rather he dealt with it all, "up close and personal" on behalf of all through the vicarious humanity of Jesus.
But when it comes to evil still in the world, there is more to be said. What is objectively and universally true of all humanity (from the "best" of us to the "worst"), is not experienced subjectively (personally) by all. Paul makes this clear in Colossians 1, where he says this:
God [the Father] was pleased to have all his fullness dwell in him [Jesus], and through him [Jesus] to reconcile to himself all things, whether things on earth or things in heaven, by making peace through his blood, shed on the cross. (Col. 1:19-20)This statement speaks to the universal and objective state of all humanity - all were reconciled through Jesus to the Father at the cross. Not "will be reconciled" but "reconciled" (past tense). As we would say, "It's a done deal!"
But there is more to be said, and Paul goes on to addresses the personal and subjective aspect of this universal picture. Note this, where Paul is speaking to Colossian believers:
Once you were alienated from God and were enemies in your minds, because of your evil behavior. (Col. 1:21)When God reconciled us to himself at the cross of Christ, all alienation he held toward us was resolved for all and forever. All of us, in Christ, are his dearly loved, forgiven, accepted and adopted children. But there remains in the minds of some (many?) an alienation toward their Heavenly Father - an alienation that spews forth in evil behavior.
Thus for all to be included in Christ, does not mean that evil has been eliminated. Yes, evil was conquered at the cross, but its complete elimination from human minds and behavior awaits the completion of God's plan to destroy all vestiges of evil - the "yeast" of the Kingdom continues to spread and to conquer - one mind at a time, until "every knee shall bow" to Christ.
Our role as believers is to declare the truly good news that God has indeed accepted, included and adopted all in and through Jesus Christ. Because of Christ, God loves them unconditionally, and the Spirit is inviting and enabling them to repent (change their thinking) concerning who God is and who they are in light of the truth of Jesus; and then to place their trust (belief) in this Jesus who has indeed saved them - picking up their cross and following Jesus as one of his disciples.
Yes the gospel truly is good news, and it deals directly and decisively with the issue of evil. It is not naive about evil. Unlike some liberal theologies, this truth does not sweep evil under the carpet, hoping that it will go away. No, Jesus has taken all evil upon himself and has and is redeeming it - transforming the perpetrators of evil into his loyal disciples. This is the transformation that is the focus of the Holy Spirit's ongoing ministry to conform us individually and collectively into the image of Christ.
[Updated 5/20/2019]