"The Hopes of the Church of God" contain eleven addresses, all of which are worth reading. But I draw attention to two that I think are extremely important to understand, if we want to grasp Darby's personal theology: "First Resurrection; or, Resurrection of the Just" and Progress of Evil on the Earth. (Yep, those are both on the STEM Publishing website.)
#5 "First Resurrection; or, Resurrection of the Just"
This is a study into the two resurrections. It demonstrates that the Scripture teaches there are two resurrections: one of the just to life, one of the unjust to judgment. Further, it demonstrates that the testimony of Scripture is consistently that these two resurrections are temporally distinct: they happen quite separately.
The resurrection of the church is a thing of itself, because the church participates in the resurrection of Christ we are raised, not only because Jesus Christ will call us from the grave, but because we are one with Him. It is by reason of this unity, that, in partaking of faith, we are already raised with Christ, raised as to the soul, but not as to the body. The justification of the church is, that it is risen with Christ. (p. 304)
Observe, in the passages concerning the resurrection, not one speaks of a simultaneous rising of just and unjust; and those which refer to the resurrection of the just speak of it always as of a thing distinct. All will rise. There will be a resurrection of the just, and a resurrection of the unjust, but they will not take place together. I will cite the passages successively, which refer to it. It is at the coming of Christ that the church will rise; Phil. 3: 20, 21; 1 Cor. 15: 23. (p. 305)
If you google "Plymouth Brethren" or "John Nelson Darby," you find a multitude of online resources attacking the idea of "secret rapture". It seems every second-year student at any given Reformed seminary has written a paper repeating the slander that JND took his idea of the return of Christ from some ecstatic utterances of a young Irvingite girl. Nonsense. Darby's view of the "secret rapture" was based in the Scriptural teaching that there are two resurrections.
Interestingly, what Darby held to be the hope of the Christian, the resurrection of the just, "brethren" themselves moved away from very quickly, talking about "rapture" where Scripture talks about "gathering together unto Him" (2 Thess. 2:1, KJV). I have found in my own experience, non-biblical terminology can be very, very dangerous.
But the repeated attacks on JND and the teaching of two distinct resurrections are alone enough to make me think he was really onto something. As he himself might point out, it is always resurrection that proved the point of disbelief in the Acts. Paul on Mars Hill was interesting to tha pagans, until he mentioned resurrection. Then some scoffed, but some wanted to hear more. Resurrection is the watershed of the Gospel: the point that divides faith from unbelief.
If the first resurrection - that of the just - is not to be taken literally, why should the second - that of the unjust - be so taken? As the object of our hope, and source of our consolation and of our joy, it is but a small thing to know that the unjust shall be raised; but the precious thing - the essential - is to know that the resurrection of the just will be the consummation of their happiness; that in it God will accomplish His love towards us; that, after having given life to our souls, He will give life to our bodies, and will make of the dust of the earth a form suitable to the life which has been given to us on the part of God. We never read in the word of God of glorified spirits, but always of glorified bodies. There is the glory of God, and the glory of those who will be raised. (p. 309)
I would say this is probably the most important prophetic paper by JND.
#4 Progress of Evil on the Earth
This paper presents Darby's understanding of the Church in Ruin. You must grasp this idea to make any sense of his ecclesiology. This is the idea he first presented in his 1828 paper "Considerations on the Nature and Unity of the Church of Christ" (Collected Writings of J. N. Darby, Vol. 1). It is this one idea that really drove "exclusive brethren" in their ecclesiology: that the Church had become apostate, and that the apostasy would not be cured until God Himself judges it at the end of the age.
What we are about to consider will tend to shew that, instead of permitting ourselves to hope for a continued progress of good, we must expect a progress of evil; and that the hope of the earth being filled with the knowledge of the Lord before the exercise of His judgment, and the consummation of this judgment on the earth, is delusive. We are to expect evil, until it becomes so flagrant that it will be necessary for the Lord to judge it. (p. 310)
This one paper contains a wealth of teaching that mainstream Dispensationalists have really neglected. I would say this one paper presents most clearly what the difference is between mainstream dispensationlism and the [more Scriptural] simpler dispensationalism of early "brethren".
In brief, the consistent dispensational approach JND took predicted a growing apostasy in the Church. Apostasy began in the Apostles' time, and really grows without remedy until the Lord judges it. He saw the presence of the church on earth as really no different than any other period of God's earthly dealings: it fails early and irrevocably, then there is judgment.
Of course this flies in the face of both Amillenianlism (including full Praeterism) and Postmillenialism: it denies that there will be any such thing as a righteous society on earth until Christ Himself establishes it after His physical return.
This is a great paper: it's well worth the 30 minutes to read. In fact, this whole collection is worth your time.