The contrast between the corruptible and the incorruptible is supported with a quotation from Isaiah 40:6-8. Isaiah describes the frailty of man and the transient nature of his life after the fall and God’s curse on the earth, and he contrasts this with the abiding nature of the word of God and the impossibility that it should pass away.
Implicit in these few verses is a doctrine that we call the perseverance of the saints. It means that if you are truly converted and you are truly saved, an internal seed has been planted in you. New life has been brought into by the Spirit of God. You will be saved and you will never be lost. Many have chosen a new description of this doctrine, and they think it's better. They call it the preservation of the saints. Why is the perseverance of the saints a much better expression of the doctrine of spiritual security? Because it teaches how the saints are preserved: by persevering. How can you tell a true convert? Simply that he is preserved? Yes, he is, by the Lord, but that can be misleading. For a believer merely to be preserved may suggest that he can do as he likes. ‘I'm saved. Once saved, always saved. I can let things slip. I can enjoy a good deal of sinful worldly living. I can live for myself, to a large extent, and for my pride and wealth and earthly enjoyment, because, after all, I cannot be lost.’ No, here is how you tell a saint: when tempted, when drawn aside, when facing hostility or persecution, when facing great discouragement or disappointment, he perseveres in faith. There is a true work of God within him, which gives him determination. The Baptist Confess of Faith of 1689 uses almost rhapsodic language: ‘Though many storms and floods arise and beat against them [believers], yet they shall never be able to take them off that foundation and rock, which by faith they are fastened upon. Notwithstanding, through unbelief and the temptation of Satan, the sensible sight of the light and love of God may for a time be clouded and obscured from them, yet he [God] is still the same, and they shall be sure to be kept by the power of God unto salvation, where they shall enjoy their purchased possession, they being engraved upon the palms of his hands and their names having been written in the book of life from all eternity.’ What a blessing it is to be saved and to be secure, and to be given the longing and the strength to persevere.