Because prayer is raising the heart and mind to God, only the sinless can pray for only they can really unite with God.
Do not forget that religion says that in so far as prayer is tainted with sin it will not do anything. So the way that works is that if you ask to become a heroic missionary you will become a good one not a heroic one. The power of prayer is limited by sin.
That encourages one to think their prayer does work despite seeming not to for sin got in the way. Imagine what that should do to a person who prays for a friend to get better and who dies in agony.
Doubts are a sin if they call God untrustworthy. If you have faith but there are no doubts, what that faith is very mild and weak? It would be no better than doubting. It says you are reluctant to trust God correctly.
Jesus says that strong faith is necessary for God to answer prayers. James says it too and James is supposed to be the word of God, scripture. Suppose you pray to heal your child. Now, that means if you pray with little faith or with doubts then you are forcing God NOT to answer your prayer and perhaps do the opposite of what you ask or do nothing which amounts to the same thing. He can't answer the prayer for he has said that he will not reward limp faith with an answer.
Some Christians say that prayer develops faith. When you find God working in you through it and answering prayers it gets stronger. You see it working and so it grows. This contradicts the doctrine that proper faith is needed before prayer has much hope of being answered. Faith must be weak to start off with. So how then could prayerdevelop it when God cannot answer the prayer of weak faith?
The man who has not tried to reduce the risk of starting to doubt by putting learning about the faith and incorporating what is learned into his relationship with God is no better than a doubter. He does not act like a person who feels that the more he learns the more he knows and develops a relationship with God. In so far as he keeps his faith less strong than it could be he is doubting. All believers are like that man to one degree or another. God does not answer the prayers of doubters. That means that all the incredible answers to prayer that most believers report are all in their heads for they are coincidence or they have done something to answer the prayers themselves. And the answers only produce hypocrisy for they are happier to get what they want from God than they would be to die for him rather than be separated from him for two seconds. St John Mary Vianney stated that we should love God so much that we would rather cease to exist than lose him even for a moment. Hypothetically, this is saying that if God could go out of existence for a second and the only way to save him was by ending life on earth then life on earth should be ended. No God is going to degrade himself by answering the prayers. If they are being answered then it is some lunatic God or Satan who is doing it. So Mary in her apparitions at Lourdes, Fatima and Medjugorje commanded prayer and told the visionaries that it would be answered. Liar.
I find the ban on boasting in the Bible to be inexplicable while Christians are allowed to boast about their faith and lack of doubt when they tell you they will pray for you! They are saying:
We are great and we have let God remove all our doubts so our prayers will be effective which is why we will pray for you�.
That is totally insulting to doubters who have good reason to doubt and it is totally pompous. It shows that the boast of the smug apostles and their Church that Jesus and the faith he preached gave nobody the right to think themselves more moral than anybody else (Ephesians 2:8-10) to have been another of their shameless multifarious deceptions.