There are really two options here:
- James is telling the poor to take pride when they experience a reversal of material fortune.
- James is telling the poor to take pride in their exaltation in Christ.
The first option would be difficult to accept, though, given the context. After all, James goes on right after to remind the rich that wealth quickly fades. Moreover, it's difficult to imagine any context in which James might make such a point. As Albert Barnes writes: "it seems not at all likely that an Apostle would exhort a poor man to rejoice in his exaltation to wealth."
The second option, on the other hand, has much to recommend it. First, if we look ahead to James 2:5 we read, "Listen, my dear brothers: Has not God chose those who are poor in the eyes of the world to be rich in faith and to inherit the kingdom he promised those who love him?" (NIV) It's not a far leap to think, then, that the "high estate" the lowly brother possesses is the richness of faith and the inheritance of the kingdom. The second thing to consider is the command to "take pride" or "boast" or "glory". Doug Moo writes about this word in his commentary (PNTC) and has this to say:
The trajectory of the term [take pride] is set decisively by the famous exhortation in Jer. 9:23-24:
Let not the wise man boast of his wisdom or the strong man boast
of his strength or the rich man boast of his riches, but let him
who boasts boast about this: that he understands and knows me,
that I am the LORD, who exercises kindness, justice and
righteousness on earth, for in these I delight,”
declares the LORD.
It seems likely then that James is encouraging the poor to look past their lowly material position and take hope in their lasting inheritance.