John Calvin Commentary Romans 8:27

"and he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to [the will of] God." — Romans 8:27 (ASV)

But he who searches hearts, etc. This is a remarkable reason for strengthening our confidence that we are heard by God when we pray through His Spirit, for He thoroughly knows our desires, just as He knows the thoughts of His own Spirit. And here, the suitability of the word know must be noticed; for it intimates that God does not regard these emotions of the Spirit as new or strange, nor does He reject them as unreasonable. Instead, He allows them and, at the same time, kindly accepts them, as allowed and approved by Him.

Just as Paul had previously testified that God aids us when He draws us, as it were, into His own bosom, so now he adds another consolation: our prayers, of which He is the director, shall by no means be disappointed. The reason is also immediately added, because He thus conforms us to His own will. It therefore follows that what is agreeable to His will, by which all things are ruled, can never be in vain. Let us also learn from this that what holds the first place in prayer is agreement with the will of the Lord, whom our wishes do not, by any means, hold under obligation. If, then, we would have our prayers be acceptable to God, we must pray that He may regulate them according to His will.