There must have been some kind of difference between the unfallen angels and the fallen angels even from their moment of creation, though it could not have been an error in their creation.
Whatever this difference was, we see from the example of the fallen angels that "It does not follow that every thing that is eternal is, therefore, blessed--for the pain of Hell is called eternal."
We also know that the unfallen angels are "not the only rational or intellectual creatures who we think should be called blessed." This because pre-fall man in the Garden was likewise blessed, as are Christians today.
And even today we rightly regard as happy all those whom we see leading a good and holy life in the hope of future immortality, untroubled in conscience and with easy access to God's forgiveness for the sins which are due to the frailty of human nature.Yet, Augustine (quite rightly) warns us against complacency in our forgiveness:
These saints, however, although certain of their reward if they persevere, can never be sure of their perseverance. For, no man can be sure that he will continue to the end to act and advance in grace unless this fact is revealed to him by God. In His just and secret counsel, God, although He never deceives anyone, gives but few assurances in this matter.To be sure, none who are saved will ever be lost. There are, however, those who are self-deceived as to the reality of their own salvation. I suspect that we all know people who seemed to be faithful believers, could clearly articulate the Gospel and appeared to delight in it, were involved and active and church and to all appearances desired to live a life of holiness. And yet, something comes along and drags them away--they reject their previously professed faith and declare that they are now enemies of God. Perhaps not in those words, but in their deeds and thoughts.
These are the people the author of Hebrews speaks of he writes:
For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.Or those whom Jesus compares to rocky soil, which initially appears to be fruitful but which ultimately does not produce.
The true believer is repeatedly called by Scripture to self-examination and warned to cling fast to the faith. At the end of the day, this and this alone is the mark of the true perseverance of the saints: that they do not reject Christ but instead hold fast and never be spoken of as John describes the same sorts of people mentioned in Hebrews:
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.If you want an excellent resource to help think more carefully about this, Jonathan Edwards' Religious Affections is written to help believers through this issue.
Again, we are told that there must have been some kind of difference in the creation of the angels that fell which made them capable of falling (though of course without attributing responsibility to God for their rebellion). These fallen angels lack the promise of eternal happiness which unfallen angels and redeemed man may look forward to.