This post doesn't come out of extensive research but just a wondering about petitionary prayer. Consider the following two scenarios:
1) When Percy learns of his wife Sally's sickness, he says a prayer for her. However, when he hears from the doctor that this sickness is life-threatening, he calls his relatives and church community, asking them to also pray for her healing.
2) Hermione says a prayer for her friend's well-being. However, Hermione desires more than anything else for her daughter's well-being in college. She goes to bed every night, asking God for this.
From what I know, many religious communities find the actions in (1) and (2) to be commonplace, normal, and even rational. (We see an analogy to persistent prayer in Jesus' parable of the woman asking the judge for justice, and we see communal prayer all throughout Acts and the epistles.) But I wonder why, exactly, more petitionary prayers are supposed to be helpful. Here are some possibilities:
a) Praying itself is virtue-building. Hence, you grow in virtue when you pray more, and others grow when they pray more. (It also grows you closer to God. Furthermore, it can grow the community closer together. I include all this under (a).)
b) When we come to God with prayer, it is a time when we are more open to God speaking to us. Hence, in asking more, there is greater chance that God will speak (either to us or the community) and we can receive guidance on what to do.
c) In praying, we end up surrendering the situations we are concerned about to God's sovereignty and will.
d) More prayer increases the objective probability that God will answer affirmatively.
All of (a) through (c) might be true. However, (b) and (c) don't seem to call for petitionary prayer. We might as well drop the petitionary prayer and instead pray prayers of surrender or prayers for guidance. Regarding (a), people who pray this way certainly don't think that virtue building is the only thing going on. Percy and Hermione certainly don't think that all they're doing is building virtue in themselves or their church. ("My wife is sick. This is a great chance for other people to grow in virtue when they pray for her!")
It seems that the activities in (1) and (2) are motivated by a belief in (d). But is (d) true? I wonder why it should be true. Why can't God just hear the one prayer and let that be enough? If I want to request for something from a friend, I only need to ask them once. Asking twice is only justified by the fact that they might not have heard me or need to be reminded, things that don't apply to God. Calling other friends to ask that friend the same thing seems unnecessary once the friend has heard from me directly. So, if God hears the request once, isn't that enough? Why should more requests make a difference to the probability of God's answering affirmatively?