Previously in Isaiah 30:25 it says, "In repentance and rest is your salvation, in quietness and trust is your strength, but you would have none of it." One must be willing to take on a faith of quiet confidence to enter into God's salvation - God's righteouness, which is being right with God. But not only so, having entered into God's righteousness something interesting happens. That quiet confidence takes on a permanent nature. It will be a quiet confidence forever. For not only once saved always saved, but also once saved it is inevitable for one's quiet confidence in God to endure forever. That is an effect or fruit of righteousness.
Conversely those who fall away indicate by doing so that they were never saved to begin with, as John gives a particular example saying, 'They went out from us, but they did not really belong to us. For if they had belonged to us [once saved], they would have remained with us [always saved]; but their going showed that none of them belonged to us." 1John 2:19Isaiah again alludes to the coming kingdom in which what is true spiritually will manifest itself physically, in contrast to the case presently in which his discipline is upon Israel and they experience tribulation.
John 16:33 "These things I have spoken to you, so that in Me you may have peace. In the world you have tribulation, but take courage; I have overcome the world."