The suffering of sickness and the suffering of persecution have this in common: they are both intended by Satan for the destruction of our faith, and governed by God for the purifying of our faith... Christ sovereignly accomplishes His loving, purifying purpose, by overruling Satan's destructive attempts. Satan is always aiming to destroy our faith; but Christ magnifies His power in weakness.
John PiperI sometimes fear that we have so redefined conversion in terms of human decisions and have so removed any necessity of the experience of God’s Spirit, that many people think they are saved when in fact they only have Christian ideas in their head not spiritual power in their heart.
John Piper