A man with a double mind is a man who is unstable in all his ways is what the scripture says. I had always thought this was referring to that man's faith in G-d to receive what he was praying for. I thought like many people who put their faith in their faith, that this man, as long as he believed and didn't doubt, he would receive what so ever he had asked for in belief.
What Is Faith?
Faith, according Hebrews 11:1 in the bible, faith is the substance of things hoped for, the evidence of things not seen.
That scripture suddenly came to me one day while I was talking to one of my sons, and as it occurred to me, I then realized that a double minded man is unstable in all his ways is also confirmed in the scripture in Proverbs...
For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.
A double minded man can sometimes try to mind the things of G-d, while also minding the things of the world, and in turn can expect to receive no reward from G-d. The double minded man will try to make things happen on his/her own instead of waiting on G-d, which requires true faith. Again, faith being the substance of things hoped for, the evidence of things not seen.
Another way to also look at double-mindedness is your average Christian trying to live their life through "faith" while not truly walking in that said faith or belief.
For as the body without the spirit is dead, so faith withoutworks is dead also.
Most Christians will spout off about One having fallen from grace if One tries to practice what is preached, by having corresponding works accompanied with their professed faith. If a person truly believes on the word of G-d, they will do as they believe, their works will correspond to their beliefs in G-d's word. Faith accompanied by the good works G-d has ordained we should walk in will cause a man/woman, to be stable in all their ways, when we lean not to our own understanding, but in all our ways we acknowledge him.