Grace works on this principle: he who does not progress, retrogresses. God gives us grace every second. This grace contains three elements: the will of God, how to do the will of God and the strength to do God’s will. God does this every second so we move towards Him every second by responding favorably to these graces.
It also works on a negative way in that Grace tells us what is contrary to the Will of God, how to avoid it and the strength to avoid it. Every second God, through His grace, tells us what good we must do and what evil we must avoid, how to go about doing each and the strength to obey His graces.
But these graces does not come every second…unless we respond favorably to it. When we reject His promptings He withholds it to encourage us to ask for it in prayer thus the necessity for unceasing prayer in asking God to respond favorably to it.
Every second God presents to us the good we must do and the evil we must avoid. Sometimes He presents to us consecutive goods to do or consecutive evils we must avoid. So we need graces to do the consecutive goods God presents to us and graces to avoid the consecutive evils we have to avoid. Since these goods and evil comes in floods and graces (to do good and avoid evil) can only from God, if you are following my reasoning we would need to pray unceasingly.
The choice between the good we must do and the evil we must avoid does not cease. It is continuous. God constantly presenting the good and the devil presenting the evil. Now comes the problem, one cannot pray unceasingly in the world with all its destruction. More often than not, man misses the good he must do (if he is without God’s grace) and does the evil he must avoid (which is natural without grace.)
In fact, in the example of Christ, he demonstrated to us how to develop the habit of unceasing prayer during His 30 years of hidden life, before he went into the world (though He had the unceasing presence of God all the time.) That is why He overcame Satan in the desert. Without this habit of prayer it is impossible to walk through the world safely. What most people have is sporadic prayer. That won’t work.
The first monks, inspired by angels, tried developing this by reciting 150 psalms a day and keeping that attitude of prayer in between the divine office. That is impossible for most people, even for most monks today. That’s why Pope Benedict keeps on repeating the importance of prayer in his addresses to the Bishop. If they don’t do it, they won’t receive the grace to know God’s will, one of which is to pray. Without knowing how to pray, how can they teach people how to get those graces. Without those graces men will find it impossible to do good and avoid evil.