I remember being a fairly new Christian and sitting in a fairly large church in Omaha. The pastor (who has since moved on) was preaching a sermon on the activity of Satan. In the midst of it he let this fly. It is pretty much a direct quote:
I always pray silently whenever I have anything really big or strategic to pray about concerning the kingdom.
I chewed on that one for quite awhile. Was he serious? Did he really believe that God’s will could be overthrown by an eavesdropping devil? Or did he believe that Satan would be frustrated by him ‘going dark’ during prayer time? Judging by his ministry, he did on both accounts. At any rate, it was all a bit much for me.
While the Bible is clear that Satan is a powerful and even dangerous foe, he is not God. He does not possess the power and attributes of God. He is not equal to God. He is a created being who is accountable to his creator.
Furthermore, he is a defeated foe because of the life, death, and resurrection of Jesus. Jesus came to destroy the work of the devil (1 Jn 3.8). He can not harm nor destroy Christians (1 Jn 5.18). We are kept and protected. His doom is sure (Rev. 20.10). Our inheritance is secure (1 Pet. 1.3-4).
So, by all means, pray out loud. This we do, not with the brazen attitude of an immature, uniformed renegade, but with a mind that has been informed by and bathed in the victory of the gospel. Boast the triumph of Immanuel and pray out loud for the continuing gospel conquest!