Satan wishes to keep the children of God in a sad, and perplexed state, because he hates Christians, and cannot devour their faith, nor pluck them out of the hand of God.
As long as Satan has a child of God confused as to whether they are true believers or just deceiving themselves, he will have them in a constant state of misery, and put off of many of their Christian duties.
It is very difficult for a person to enjoy Christ, rejoice in Him, commune with Him, fellowship with the saints, pray and meditate on His word without vain interruptions, and many other ordinances when they are not even sure if they are just bringing a greater judgment on themselves by deceiving themselves as false believers or not.
And one of Satan’s ways of keeping a child of God in this confused and sad state is by having them compare their portion or degree of faith with a person in the body who appears to have a significantly greater portion or degree of faith, and then says “your faith is not like their faith, and so your faith is not a true faith. They stand firm in the same afflictions and temptations where you stumble and sin. You are not just weak, you are a phony.”
But, we must keep in mind that God distributes His grace to each Christian at the measure He sees fit (Ephesians 4:7), and gives each person a different portion of faith (Romans 12:3), just as He does with knowledge and wisdom, and so on. All for His glory.
A light from a candle may not be as bright as the light of a bonfire, but it is still light. We would not let Satan tell us the candle has no light just because it isn’t as bright as the bonfire, and we should not believe him when comparing weaker faith to greater portions of faith.
Our reply should be “Yes, my faith may only appear to be the size of a mustard seed right now, but my great God planted it in me, and He didn’t do so in vain. It is small, but sincere and equally as true and saving as the faith the size of an entire mustard tree.” --Joshua Arnold