Christ promises, "If you abide in me, and my words abide in you, ask whatever you wish, and it will be done for you" (John 15:7).
When you first read "ask whatever you wish, and it will be done for you," you can kind of tense up with a mental knee-jerk reaction, "Well, not WHATEVER you wish, of course." But if you understand the context of John 15, there should be no discomfort from Christ's promise to do for you whatever you wish, if you abide in Him and He in you. Let me explain:
John 15:5 says, "Whoever abides (remains) in me and I in him, he it is that bears much fruit." And not too much later, John 15:8 says, "By this the Father is glorified, that you bear much fruit and so prove to be my disciples."
John teaches us here that God is glorified in the lives of His disciples when they bear much fruit. And the only way to bear much fruit is to abide in Christ and He in us. So logically, the more Christ is seen in and through us, the more God is glorified. The reason for this is that there is no one or no thing that brings God more pleasure than His Son (Mark 1:11 "And a voice came from heaven, 'You are my beloved Son; with you I am well pleased.'").
So if you have trusted in Christ and been united with Him, what do you think God would desire to do most with your life? Well, His primary concern would be to do whatever would bring Him most glory, because he alone is worthy (Psalm 29:2). And the means by which He brings glory to Himself through your life is by the flourishing of the character of Christ in you. Our lives become a conduit, or channel, or "branch" through which the Father achieves His ultimate end of glorifying Himself by bringing His divine life through His Son, "the true vine," to bear much fruit in us.
Return now to the promise in verse 7: "ask whatever you wish, and it will be done for you." If your will is completely subject to God's as you abide in Christ, then you will only be asking God for things that bring glory to Him; things that enable you to bear much fruit, and so feature the life of Christ.
In the words of the psalmist: "Delight yourself in the LORD, and He will give you the desires of your heart" (Psalm 37:4).
Delighting yourself in the LORD = abiding in Christ
