I pray that you may be active in sharing your faith, so that you will have a full understanding of every good thing we have in Christ.As I commented there, there are substantial improvements in the TNIV version of this verse:
I pray that your partnership with us in the faith may be effective in deepening your understanding of every good thing we share for the sake of Christ.
- NIV wrongly suggests that Philemon is not yet properly "sharing [his] faith" and that Paul is praying that he should do so. TNIV rightly clarifies that Philemon already has this.
- NIV is often understood as referring to evangelism, at least when read by evangelical Christians for whom "sharing your faith" is jargon for evangelism. It is unclear to me whether the NIV translators really intended this verse to refer to evangelism, but it seems rather unlikely that this was Paul's meaning. And so TNIV is correct to avoid this evangelical jargon and use words which point to a more probable understanding of the Greek text.
- NIV's "full understanding" goes beyond what the Greek text says, and suggests that we humans can aspire to completely understand matters which are surely beyond our full understanding. TNIV's "deepening your understanding" is much better.
- NIV closes with a rather bland "we have in Christ", for a literal "(which is) in us into Christ". This is not even the regular "in Christ" phrase, so there is little excuse for using this difficult to understand wording. Also it fails to note that the Greek word en here probably means "among" rather than "in". The TNIV rendering "we share for the sake of Christ" seems to capture the main point far more clearly.