The Christian’s assurance of salvation is sort of like the moon. Sometimes it’s full and bright. Other times it’s hidden behind dark clouds. It can even be just a sliver in the night sky. Our assurance of salvation waxes and wanes. Speaking of assurance, John Newton made some great observations on it when he preached from Job 19:25-26. Here’s a summarized and edited version of his remarks.
I believe that true assurance is not entirely due to the strength of our faith. It is also due to the fact that we have faint and weak views of some spiritual truths. If we had a more powerful impression of these truths, even the strongest assurance would totter unless our faith was strengthened at the same time.
Let me explain. If I told you that I myself do not doubt that my interest in the Gospel leads me to believe that God accepts me and will preserve me, you might then think my faith is strong. However, I have a very slight understanding of the evil of sin, of the deceitfulness of my own heart, of the force and subtlety of my spiritual enemies, of the strictness and spirituality of the law, or of the awful majesty and holiness of our great God. If God would be pleased to impress these solemn realities upon my mind with a conviction ten times greater than I have ever known (which would still be vastly short of the truth), you would probably see my countenance change and my speech falter – unless God also gave me a tenfold clearer and more powerful discovery of the grace and glory of Jesus.
The Lord, in compassion to our weakness, shows us these things, little by little, as we are able to bear them. And if, as we advance in the knowledge of ourselves and of our dangers, our knowledge of the unsearchable riches of Christ advances equally, we may rejoice in hope and we may even possess an assured hope.
How far our assurance is solid and true may be estimated by the effects. It will surely make us humble, spiritual, peaceful, and patient. I pity those who talk confidently of their hope, as if they were out of the reach of doubts and fears, while their tempers are unsanctified and their hearts are visibly attached to the love of this present world. I fear they know but little of what they say. I am better pleased when persons of this character complain of doubts and darkness. It proves at least that they are not destitute of feeling, nor, as of yet, lulled into a spirit of careless security.
If you can find this sermon by Newton called “Job’s Faith and Expectation,” I highly recommend it, especially the remarks he makes on assurance of faith. It is right and proper to long for an assured faith that says with Job, As for me, I know that my Redeemer lives, and at the last He will take his stand on the earth. Even after my skin is destroyed, yet from my flesh I shall see God” (Job 19:25-26 NASB).
(The above edited/summarized quotes are found in John Newton, Works Volume IV, p. 441ff.)