In fine, supplication for pardon, with humble and ingenuous confession of guilt, forms both the preparation and commencement of right prayer. For the holiest of men cannot hope to obtain any thing from God until he has been freely reconciled to him. God cannot be propitious to any but those whom he pardons. Hence it is not strange that this is the key by which believers open the door of prayer, as we learn from several passages in The Psalms. David, when presenting a request on a different subject, says, "Remember not the sins of my youth, nor my transgressions; according to thy mercy remember me, for thy goodness sake, O Lord," (Psalm 25:7.) Again, "Look upon my affliction and my pain, and forgive my sins," (Psalm 25:18.) Here also we see that it is not sufficient to call ourselves to account for the sins of each passing day; we must also call to mind those which might seem to have been long before buried in oblivion.
... prayers will never reach God unless they are founded on free mercy. To this we may refer the words of John, "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness," (1 John 1:9.) Hence, under the law it was necessary to consecrate prayers by the expiation of blood, both that they might be accepted, and that the people might be warned that they were unworthy of the high privilege until, being purged from their defilements, they founded their confidence in prayer entirely on the mercy of God.John Calvin, Institutes of Religion: Book 3.20.9
Interestingly, it is not union with Christ, either elective or spiritual, that Calvin highlights as the ground upon which our prayers are received by God. The reason, it seems, is that union with Christ as a doctrine fails to address the central need of the saved-yet-still-sinner as he comes to God in prayer. That need is for assurance and confidence in order to approach the living God. "... how much more shall the blood of Christ, who through the eternal Spirit offered himself without blemish unto God, cleanse your conscience from dead works to serve the living God?" (Hebrews 9:14) Our prayers are heard of God because of the pardon of sins found only in the meritorious blood of Christ which apprehended by faith gives believers a firm ground of confidence upon which to pray. "Let us therefore draw near with boldness unto the throne of grace, that we may receive mercy, and may find grace to help us in time of need." (Hebrews 4:16)