Not even all devils ...

would wrest this passage from me.

This is what Calvin wrote about Philippians 2:6,

6. Who, being in the form of God, thought it not robbery to be equal with God:

6. Qui quum in forma Dei esset, non rapinam arbitratus esset, Deo aequalem se esse:

In the Latin Vulgate, the Greek word harpagmos, ἁρπαγμός, had been translated as rapina, meaning "rape, pillage, plunder and robbery." This is how Luther and Calvin also understood this word harpagmos. Calvin wrote,

Thought it not robbery. There would have been no wrong done though he had shewn himself to be equal with God. For when he says, he would not have thought, it is as though he had said, “He knew, indeed, that this was lawful and right for him,” that we might know that his abasement was voluntary, not of necessity. ....

For where can there be equality with God without robbery, excepting only where there is the essence of God; for God always remains the same, who cries by Isaiah, I live; I will not give my glory to another. (Isa 48:11.) Form means figure or appearance, as they commonly speak. This, too, I readily grant; but will there be found, apart from God, such a form, so as to be neither false nor forged?

As, then, God is known by means of his excellences, and his works are evidences of his eternal Godhead, (Ro 1:20,) so Christ’s divine essence is rightly proved from Christ’s majesty, which he possessed equally with the Father before he humbled himself. As to myself, at least, not even all devils would wrest this passage from me — inasmuch as there is in God a most solid argument, from his glory to his essence, which are two things that are inseparable.

However, at least since the RSV, harpagmos has been translated as "a thing to be grasped." This phrase occurs in the NIV 1984, NASB and ESV. The NRSV, on the other hand, has translated harpagmos as "something to be exploited" and the NIV 2011 as "something to be used to his own advantage."

The significant thing here is that some theologians writing for the Council of Biblical Manhood and Womanhood have interpreted "did not count equality with God a thing to be grasped" as an indication that Christ is in some way not equal to God. Here Denny Burk writes for the CBMW,

First, this verse affirms that Christ has ontological equality with the Father with respect to his deity. That's what "existing in the form of God" means. Second, the verse affirms that in his pre-incarnate state Christ did not try to obtain (or "grasp for") another kind of equality which he did not have in his pre-existent state.

What kind of "equality" did he refuse to grasp for? He refused to "grasp for" a functional equality with the Father that would have usurped the Father's role as Father. In contrast to grasping for that kind of equality, the Son "emptied himself" and took the form of a servant (v. 7). In other words, in eternity past Christ determined not to usurp the Father's role but decided to embrace his own role in the incarnation. Thus what we have in this text is both an affirmation of Christ's ontological equality with the Father (vis a vis his deity) and a passing reference to his functional distinction from the same.

While Burk says that Christ did not have a certain kind of equality with God, Calvin writes that this means that Christ can be equal to God without robbery. For Burk, Christ is ontologically equal but functionally distinct, or "not equal"; and for Calvin, Christ's essence and his majesty, (in my view this means his authority and power) are inseparable, and Christ is equal to God in both.

If you wish to undestand Calvin and Luther, if you wish to feel close to the original Greek, then you could do a great deal worse than read the King James Versions,

Who, being in the form of God, thought it not robbery to be equal with God:

You might also want to dip into the translation of the Brethren scholar, J. N. Darby,

who, subsisting in the form of God, did not esteem it an object of rapine to be on an equality with God;

Dear Reader,

If you want both heritage and transformation, if you value traditional Christianity, if you are curious about the religion of the Reformers, of the orthodox church through the ages, read the King James Version. I am delighted to see, however, that in many places, the NIV 2011 has returned to the original meaning of the King James Version, closer to a literal understanding of the Greek, than the NIV1984.

I write with feeling about this passage, as it was one of two passages which my grandfather had the habit of reading out loud in the meeting. He would stand and read Phil. 2, wavering back and forth as he stood, Bible open, reciting this chapter from memory. We all value the Bible we had from our childhood, the Bible in our own heart language.

For more on this passage, read the post at Biblegateway by Craig Blomberg.