Pope Shenouda III
  
Conditions of the  acceptable prayer and exercises in prayer
  Not each prayer is acceptable because not each prayer is a PRAYER.
  The prayer of the proud   Pharisee  was not  acceptable like the prayer of the contrite tax collector, who  returned  to his house forgiven (Lk18:14). The same with the prayer of  those  whose hands are full of blood. The Lord said about them, “When you  spread out your hands, I will hide My eyes from you; Even though you  make many prayers, I will not hear” (Is 1:15 ). And also the prayer of the hypocrites (Mt 6), and those  who for a pretence make long prayers (Mt 23:14).
 You might pray a prayer and one of the  twenty four  elders would proceed and take it in his golden censor and  present it to  God, a sweet aroma of incense... (Rev 5:8). While another  person would  pray all day long and the angels would wonder at him, as  none of his  prayers ascended!