Why do believers need God to open their hearts and give understanding?
Believers need God to open their hearts and give understanding because Satan, the authority over principalities and powers, actively works to frustrate their lives and keep lost humanity in darkness. Only God, through the Holy Spirit, can break that power and grant understanding.
Les explains that believers wrestle not against flesh and blood but against principalities and powers whose authority is Satan. Satan does everything he can to frustrate the life of believers and to keep lost humanity in darkness, and he has that power until God breaks it. This is why believers must come back to the fact that God is the One who opens hearts, and God is the One through the working of the Holy Spirit who gives understanding. Paul later assures believers in Romans 8 that neither principalities, powers, things present, things to come, height, depth, nor any other creature can separate us from the love of God in Christ Jesus. Even when someone rejects Paul, the same truth is confirmed by Jesus Himself in John's Gospel.