Much discussion has come forth over the past several weeks in the biblical blogosphere regarding the πιστις Χριστου debate. Now, for a quick summary, read this post.
The discussion has rested heavily in Romans. The general consensus: πιστις Χριστου is objective (faith in Christ) and not subjective (Christ’s faith/faithfulness). I want to enter the debate with Ephesians in mind.
NT Wrong briefly mentions Ephesians in the debate on his blog by referencing Eph 3:12 and 4:13 in passing.
In regards to 3:12, I have already stated my position, that the faith in question is part of a subjective genitive clause, meaning it is Christ’s faithfulness that provides us with access to God.
The general discussion throughout the blogosphere has maintained a position that disregards the subjective genitive as a possibility in favor of the objective genitive. There is good reason for this position, especially in regards to Romans. But in Ephesians 3:12, something else is going on. What may be true in Romans is not true everywhere else in the New Testament necessarily.
The clause in question in Ephesians 3:12 is entirely genitive. Additionally, Christ is not explicitly mentioned; instead, there is a reference to “him.” Furthermore, the clause is part of a prepositional phrase (δια της πιστεως αυτου). A similar construction is used in 3:17, only δια της πιστεως is followed by a dative prepositional phrase (εν ταις καρδιαις υμων). The latter is clearly referencing faith in Christ (by our faith, Christ dwells in our hearts). It is the use of the genitive in 3:12 that I want to focus on henceforth.
The former genitive construction, “through the faith in him,” rests in the context of Eph. 3:1-13. The discussion in this section continues Paul’s praise of God with respect to the work that he has done through Jesus Christ on our behalf. In Eph. 3, Paul mentions the mystery of God’s “economy of grace” (i.e., plan of salvation, household laws of grace, or however else you wish to understand την οικονομιαν της χαριτος). Paul has been given insight into this mystery; this knowledge has been hidden until the time of the apostles and prophets in the Spirit. What was the mystery? The mystery was that God had planned to bring the Judeans and the non-Judeans together into the same body as the people of God. Paul understood that he was given the task to preach this insight to the non-Judeans. This discussion is entirely focused on God’s work in Christ and does not have the faith of the saints in view.
The previous section, Eph. 2:11-22, also focuses on God’s work in Christ to bring the Judeans and the non-Judeans together into one temple. The first section, Ephesians 2:1-10, although it does say that it is by faith (it is implied that it is the faith of the saints) we are being saved, the focus of this passage is God’s work in Christ to raise the people to life. In the great eulogy (Eph. 1:3-14) and the propositio (Eph. 1:15-23), the focus rests on God’s work in Christ; Paul does mention the faith of the saints in Jesus, but he is not focused on their faith.
The focus of Ephesians is not our faith, but rather, God’s work in Christ. Bear in mind that if Christ had not remained faithful, we would not have access to God, we would not be made into a new person or temple, and we would not have grace. Additionally, Eph. 1:17 suggests that Christ did have faith in the Father, for it says, “. . . in order that the God of our Lord Jesus Christ, the Father of glory, might give to you a spirit of wisdom and revelation in his knowledge, . . .” Christ has a God–God the Father. Christ remained faithful to God, as is evident in the Garden of Gethsemane. Therefore, it is possible to speak of Christ’s faith.
Since Ephesians focuses on God’s work in Christ and it is possible to speak of Christ’s faith, we are theologically and contextually capable of seeing a subjective genitive in Eph. 3:12. The use of αὐτοῦ in Ephesians is nearly always subjective or possessive when it is connected with a noun; when used in the same genitive prepositional phrase, δια + genitive article + genitive noun + αὐτοῦ, which occurs in Eph. 1:7 and 3:16, αὐτοῦ functions possessively. The style of Ephesians also suggests a subjective use of αὐτοῦ in Eph. 3:12. When we consider the theological and literary context along with the letter’s style, it is best to see a subjective use in Eph. 3:12.
Every case of the πιστις Χριστου debate needs to be considered in its own context. What happens in Romans may not be true necessarily for Ephesians and vice versa. In the debate, it seems that Eph. 3:12 is at least one case where a subjective genitive should be taken over an objective genitive.
 I have not read all of NT Wrong’s blog posts. After performing a quick search on his blog, I did not find any posts after his original entry that deal with the δια της πιστεως αυτου phrase in Eph. 3:12. Searching his blog is not easy, especially since I could not find a search field on his blog.
 The following quotation comes from my explanation of Eph. 3:8-12, and it gives my reasons for translating the genitive noun phrase in v. 12 as a subjective genitive:
Many translations take the last genitive phrase, διὰ τῆς πίστεως αὐτοῦ, as an objective genitive. They translate it, through faith in him. However, we cannot translate it as an objective genitive. Instead, we must see it as a subjective genitive, and we need to translate it, through his faith. There are several pieces pointing to the subjective use rather than the objective use in Ephesians. The use of αὐτοῦ in Ephesians is always subjective or possessive. It is used a total of 33 times as one of these options, but never as an objective genitive. The use of διὰ plus the genitive article plus a genitive noun plus αὐτοῦ occurs in two other places in Ephesians, 1:7 and 3:16, and in both of those cases αὐτοῦ functions subjectively (first, through his blood, and second, through his Spirit). These reasons are enough to indicate at the least that an objective use of αὐτοῦ is not found in Ephesians, so it would be very strange for Paul to shift and use it in 3:12. Given the use of the genitive construction elsewhere and the use of αὐτοῦ throughout Ephesians, we conclude that this phrase is to be taken as a subjective genitive, translating it, through his faith
There is one place where αὐτοῦ might be objective, and that is found in Eph. 1:17. Additionally, to be more precise, αὐτοῦ does not always function subjectively or possessively. There are a couple of instances in which it functions differently, but it is important that, when it is not subjective or possessive, it is not objective, with perhaps Eph. 1:17 being the one exception. I take αὐτοῦ in this case to refer to God’s knowledge, the content of which is found in .vv 18-19. God’s knowledge is a knowledge of his calling, his inheritance for the saints, and his power. Paul is praying that God will reveal his knowledge (calling, inheritance, and power) to his readers. I admit that I am alone in this understanding of Eph. 1:17.