James 1:8

A double-minded man is unstable in all his ways.
James 1:8 Modern English Version (MEV)
 
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A double minded man - The word here used, δίψυχος dipsuchos occurs only here and in Jas_4:8. It means, properly, one who has two souls; then one who is wavering or inconstant. It is applicable to a man who has no settled principles; who is controlled by passion; who is influenced by popular feeling; who is now inclined to one opinion or course of conduct, and now to another.
 
Is unstable in all his ways - That is, not merely in regard to prayer, the point particularly under discussion, but in respect to everything. From the instability which the wavering must evince in regard to prayer, the apostle takes occasion to make the general remark concerning such a man, that stability and firmness could be expected on no subject. The hesitancy which manifested on that one subject would extend to all; and we might expect to find such a man irresolute and undetermined in all things. This is always true. If we find a man who takes hold of the promises of God with firmness; who feels the deepest assurance when he prays that God will hear prayer; who always goes to him without hesitation in his perplexities and trials, never wavering, we shall find one who is firm in his principles, steady in his integrity, settled in his determinations, and steadfast in his plans of life - a man whose character we shall feel that we understand, and in whom we can confide. Such a man eminently was Luther; and the spirit which is thus evinced by taking firmly hold of the promises of God is the best kind of religion.
 
Albert Barnes’ Notes on the Bible