"We know these books to be canonical, and the sure rule of our faith, not so much by the common accord and consent of the Church, as by the testimony and inward illumination of the Holy Spirit, which enables us to distinguish them from other ecclesiastical books upon which, however useful, we can not found any articles of faith." (Article IV)
John Calvin, in his work Institutes of the Christian Religion, again gives the same argument:
"Profane men think that religion rests only on opinion, and, therefore, that they may not believe foolishly, or on slight grounds, desire and insist to have it proved by reason that Moses and the prophets were divinely inspired. But I answer, that the testimony of the Spirit is superior to reason. For as God alone can properly bear witness to his own words, so these words will not obtain full credit in the hearts of men, until they are sealed by the inward testimony of the Spirit. The same Spirit, therefore, who spoke by the mouth of the prophets, must penetrate our hearts, in order to convince us that they faithfully delivered the message with which they were divinely entrusted." (ch 7, paragraph 3)
Prior to the Reformation, the 73 book Catholic biblical canon had been the undisputed biblical canon for over 1000 years. This new 66 book Protestant canon being defended by John Calvin had never existed before, never having been put forth by any individual or group in the 1500 years prior (even during the first three centuries of the Church during which the canon was a disputed matter). The 16th century self-appointed reformers literally removed books from the universally accepted Bible to create a brand new canon and then justified it by appealing to an "inward illumination of the Holy Spirit".
Would evangelicals accept someone doing the same thing today?
If the mega-church responded that they had made their decision "by the testimony and inward illumination of the Holy Spirit", would any evangelical be satisfied by that response? Of course not. Evangelicals would probably brand the mega-church as heretical (as was the case in the recent Rob Bell scandal).
Evangelicals might counter that the Holy Spirit wasn't leading them to remove those books. But since there's not necessarily any reason to think that the mega-church is or is not being led by the Holy Spirit any better than other evangelicals, such a counter would be useless in settling the matter (remember, priesthood of all believers).
And since Esther isn't quoted as an authority by any other books of the Bible, there can be no appeal to the other Scriptures (even if it was quoted by another book, the mega-church could just respond that that book must not be canonical as well).
Evangelicals would only be left with an appeal to tradition (again, as with Rob Bell), but such an appeal wouldn't work too well: Esther's canonicity was frequently contested in the early church and only settled when the Catholic Church settled the canon at the end of the 4th century. But when the Catholic Church did that, she established the very 73 book canon that evangelicals themselves follow the Reformers in denying. In other words, evangelicals couldn't appeal to the Catholic Church's establishment of Esther as canonical without also accepting the Church's establishment of Tobit, Judith, 1 Maccabees, 2 Maccabees, Sirach, Baruch, and Wisdom as canonical, as well as the Church's authority to settle the canon at all. Since they probably wouldn't do either of those things, evangelicals would be forced to appeal to the mere consensus among the Reformers in the 16th century (only 500 years ago, not very long in the history of Christianity) on the 66 book Protestant canon.
But even that wouldn't solve the problem. The mega-church could make an argument similar to the one made by the Reformers in the 16th century: 'We reject the authority of any church bodies or leaders. We don't follow tradition, councils, confessions, or even common consensus, but instead follow the inward illumination of the Holy Spirit which is leading us to our 65 book canon.'
If evangelicals today wouldn't allow someone to create a new Bible because they felt they were "illumined" to do so by the Holy Spirit, then evangelicals cannot go on using the 66 book Protestant canon that was created on the same grounds.*
*At least according to John Calvin, whom many evangelicals today follow and pattern themselves after