The knowledge of our union with Christ...gives us confidence in prayer. It was when Jesus had begun to expound the closeness of this union that he also began to introduce the disciples to the true heart of prayer. If Christ abides in us and we abide in him, as his word dwells in us, and we pray in his name, that God hears us (Jn 15:4-7). But all of these expressions are simply extensions of the one fundamental idea: If I am united to Christ, then all that is his is mine. So long as my heart, will and mind are one with Christ's in his word, I can approach God with the humble confidence that my prayers will be heard and answered.
Sinclair B. Ferguson, The Christian Life: A Doctrinal Introduction
prayer, union-with-christ