1 John 5:13-15
God Hears Us
13 I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life. 14 This is the confidence we have in approaching God: that if we ask anything according to his will, he hears us. 15 And if we know that he hears us—whatever we ask—we know that we have what we asked of him.
St. John writes that if we believe in the Son of God we will have eternal life...If we seek the Father according to His will, he hears us...
So, when we pray God's will be done, and it is His will, we know that what we have ask will be done...