Matthew 26:39

"Have faith in God," Jesus answered. "I tell you the truth, if anyone says to this mountain, 'Go, throw yourself into the sea,' and does not doubt in his heart but believes that what he says will happen, it will be done for him. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours.

(Mark 11:22-24)

He replied, "Because you have so little faith. I tell you the truth, if you have faith as small as a mustard seed, you can say to this mountain, 'Move from here to there' and it will move. Nothing will be impossible for you."

(Matthew 17:20)

We often use verses like these when praying. It's true that we have to pray in faith. Praying half-heartedly probably gives half-hearted results. Can you imagine a child asking their parent for something, but not really believing it can happen? Sure, it's possible for the parents to still give it to the child, but it's probably less likely. At the very least, the parents are probably less enthused about giving what the child asks for if they ask half-heartedly. I believe we should ask believing we will receive it. Clearly we should according to what Jesus said.

Is any one of you in trouble? He should pray. Is anyone happy? Let him sing songs of praise. Is any one of you sick? He should call the elders of the church to pray over him and anoint him with oil in the name of the Lord. And the prayer offered in faith will make the sick person well; the Lord will raise him up. If he has sinned, he will be forgiven. Therefore confess your sins to each other and pray for each other so that you may be healed. The prayer of a righteous man is powerful and effective.

(James 5:13-16)

Praying in faith applies to healing as well. We also have this other verse about elders praying and anointing the sick. However, when praying for things, some people argue that praying "if it is in your will" is not really praying believing it will happen. It's like asking for something then taking a step back from full belief in your request. As a result, some argue that you go against the type of praying in faith mentioned in the first two verses.

Going a little farther, he fell with his face to the ground and prayed, "My Father, if it is possible, may this cup be taken from me. Yet not as I will, but as you will."

(Matthew 26:39)

In this passage, Jesus is asking for the cup to be taken from him. However, he adds the "back step". Is it in fact possible by God? I believe so. Is Jesus praying believing that his request is entirely possible by God? I believe so. You can take this "back step" and still believe in full faith that your request is possible by God. Fine, believing you have received it is different than believing it is possible. I still think praying and asking the way Jesus did is okay. Some may disagree, but that's okay. By doing this you are acknowledging that God is able and is bigger than your request.