The Bible offers a lot of promises, but there may be none more extraordinary than the one in I John 5:14-15:
“And this is the confidence (the assurance, the privilege of boldness) which we have in Him: [we are sure] that if we ask anything (make any request) according to His will (in agreement with His own plan), He listens to and hears us. And if (since) we [positively] know that He listens to us in whatever we ask, we also know [with settled and absolute knowledge] that we have [granted us as our present possessions] the requests made of Him.”
That is from the Amplified Bible, which throws in lots of clarifications to make sure we really get the point. Here it is from the NIV, the stripped-down version:
“This is the confidence we have in approaching God: that if we ask anything according to his will, he hears us. And if we know that he hears us—whatever we ask—we know that we have what we asked of him.”
The Message gives us the same point but with updated language: “And how bold and free we then become in his presence, freely asking according to his will, sure that he’s listening. And if we’re confident that he’s listening, we know that what we’ve asked for is as good as ours.”
There is one inviolate phrase that all three versions leave intact: “according to His will.” Whatever we ask according to His will, we can be sure that He hears us and will grant us that request. How do we know what His will is? There are two ways. One is to make a concerted effort of Bible study until the assurance grows within you that you understand God’s will for you. The other is to put these verses into practice: Ask God in regular prayer what His will is for you, and when you feel the assurance that He has heard you, and has responded to you, then adjust your life accordingly.