Stumbling over the little word ἐπὶ

Today I was discoursing with some friends on good works.  It was noted early on that "work" had become a "four-letter word" to most people, because of the Fall and the Curse.  For this very reason, it seems even a great many Christians are theologically disposed against teachings of the Scriptures that center on good works.  Being among staunch Calvinists, the stress kept getting placed on how Christ has finished our work, and no work of ours can grant us any standing at all with God, as Paul clearly states, [Eph 2:8,9]

For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast.

I totally agree, as did we all, that salvation is entirely God's gift. But lest some should persist in the wrong view that we don't have to do any good work, I brought up the next verse, which stresses that good works are the purpose for which God has saved us. The NKJV has,

For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand that we should walk in them.

It couldn't be any clearer.  Enter the Greek scholar.  "No", says he. "You are mistaken, along with all those who have ever translated this verse. The word ἐπὶ in this verse means 'upon' not 'for'.  It is a mistranslation.  We are created upon HIS good works.  It does not teach that we are created 'for the purpose of' good works."

"But what about the second half of the verse? Doesn't that mean God prepared these good works for us to walk in, that is, to do?"

"Yes, that part teaches good works, but not the first part."

So now, because of the word ἐπὶ, the clearest verse teaching that we are to work is eviscerated by this rigid interpretation of a Greek preposition by a religious professional. He even asserted (incorrectly) that ἐπὶ is nowhere else translated as "for".  But a quick glance shows that epi certainly can mean "for". Luke 4:43 [KJV] says,

And he said unto them, I must preach the kingdom of God to other cities also: for therefore am I sent.

This verse contains the word ἐπὶ, translated "because" in the KJ, and "for" in most other places. 
The ESV renders it as "for I was sent for this purpose." Darby's translation is cumbersome, but crystal clear, "for for this I have been sent forth."

I'm sure he meant well.  Yes, many falsely believe their good works gain them standing with God.  And we must resist this. But not to the point of re-translating the scriptures in accordance with our theological pet peeves.

So taken aback was I at this preacher's confidence in his own knowledge of Greek prepositions, and of his disagreement with ALL translators on the wording of Eph 2:10, that I came a little unglued and launched into a tirade about people getting hung up on the salvation by works issue to the degree that they dismiss ALL good works as unnecessary, which is essentially the heresy of Antinomianism.  The tirade was pointless and a bit irrational, and offended the group.  My bad.

But what else but a pet peeve would lead a theologian to such an unconventional and senseless translation of epi here?  My first thought was, well, if Paul was saying "we are created in Chris Jesus upon His good works," surely he would have used "His" there, not left it so ambiguous.  Paul was never careless in his epistles, and most particularly Ephesians.  To add "His" is totally unacceptable, even though, theologically we certainly ARE saved by His work.  But HERE Paul is addressing why we are saved, not how:  to do good works, to "walk in them".  My friend's translation was really interpretation, at least moreso than using just "for" good works, as every other translator has done.

Though flustered a bit at the moment, I can't accept this line of his, that ἐπὶ can only mean "on" and not "for", and thus that we all have misunderstood Eph 2;10.  I'm happy to entertain new interpretations, but this one doesn't work for me.   And thankfully, I am not alone. Experts, no matter how highly-credentialed, are not always correct.  Fortunately, when an expert goes entirely against the consensus of all the other experts, their error is easy to spot.

In context with the teachings of Paul in Ephesians, it is perfectly clear that he intended us to believe we are saved for a purpose, and part of that purpose is doing good works in Christ Jesus, of ministry, of fellowship, and of holiness and obedience to the moral law of the Bible, all of which bring glory to God, which is the ultimate purpose of God in creation [Jonathan Edwards], as well as the deepest desire of those who truly are new creations in Christ.

My own reaction to this brother's presumption was poor, perhaps even carnal.  And, admittedly, I have probably caused others to stumble in the very same way, assuming I know enough to challenge anything and everything.  Lord, grant me the humility to interpret your word in such a way as to obey it fully, never to justify my lack of holiness or willingness to engage the work you have apportioned for me, and the humility to teach so that others do not stumble because of me. IJNA