6 But let him ask in faith,with no doubting, for the one who doubts is like a wave of the sea that is driven and tossed by the wind. 7 For that person must not suppose that he will receive anything from the Lord; 8 he is a double-minded man,unstable in all his ways.
Double-minded in verse 8 means two souled. A person with two mindsets, two affections, lacks a believing heart. This person needs needs to repent of their doubt and wholly trust in God.
Possessing a believing heart allows a Christian to endure trials because he understands that God is sovereign and will lovingly allow a believer's faith to be tested in order to be conformed into the image of His Son, Jesus.
Yours In Christ,
Tom Vazquez