John Calvin Commentary Romans 15:13

"Now the God of hope fill you with all joy and peace in believing, that ye may abound in hope, in the power of the Holy Spirit." — Romans 15:13 (ASV)

And may the God, etc. He now concludes the passage, as before, with prayer, in which he desires the Lord to give them whatever he had commanded. From this it appears that the Lord does in no way measure his precepts according to our strength or the power of free will, and that he does not command what we ought to do so that we, relying on our own power, might prepare ourselves to render obedience. Instead, he commands those things which require the aid of his grace, so that he may prompt us to prayer.

In saying the God of hope, he had in view the previous verse, as though he said, “May then the God in whom we all hope fill you with joy (that is, with cheerfulness of heart) and also with unity and concord, and this by believing:” for so that our peace may be approved by God, we must be bound together by real and genuine faith.

If anyone prefers to understand in believing as in order to believe, the sense will be — that they were to cultivate peace for the purpose of believing. For only then are we rightly prepared to believe, when we, being peaceable and unanimous, willingly embrace what we are taught. However, it is preferable that faith should be connected with peace and joy, for it is the bond of holy and legitimate concord and the support of godly joy. And though the peace which one has inwardly with God may also be understood, the context leads us instead to the former explanation.

He further adds, that you may abound in hope; for in this way hope is also confirmed and increased in us. The words, through the power of the Holy Spirit, indicate that all things are the gifts of divine bounty. And the word power is emphatically intended to convey that wonderful energy by which the Spirit works in us faith, hope, joy, and peace.