John Calvin on 1 John 5:15

15 And if we know This is not a superfluous repetition, as it seems to be; for what the Apostle declared in general respecting the success of prayer, he now affirms in a special manner that the godly pray or ask for nothing from God but what they obtain. But when he says that all the petitions of the faithful are heard, he speaks of right and humble petitions, and such as are consistent with the rule of obedience. For the faithful do not give loose reins to their desires, nor indulge in anything that may please them, but always regard in their prayers what God commands.

This, then, is an application of the general doctrine to the special and private benefit of every one, lest the faithful should doubt that God is propitious to prayers of each individual, so that with quiet minds they may wait until the Lord should perform what they pray for, and that being thus relieved from all trouble and anxiety, they may cast on God the burden of their cares. This ease and security ought not, however, to abate in them their earnestness in prayer, for he who is certain of a happy event ought not to abstain from praying to God. For the certainty of faith by no means generates indifference or sloth. The Apostle meant; that every one should be tranquil in these necessities when he has deposited his sighs in the bosom of God.

1 John 5:16-18

16. If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.

16. Si quis viderit fratrem suum peccantem peccato non ad mortem, petet; et dabit illi vitam peecanti, dico, non ad mortem: est peccatum ad mortem; non pro illo, dico, ut quis roget.

17. All unrighteousness is sin: and there is a sin not unto death.

17. Omnis injustitia peccatum est; et est peccatum non ad mortem.

18. We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.

18. Novlinus quod quisquis ex Deo genitus est, non peccat; sed qui genitus est ex Deo servat seipsum, et malignus non tangit eum.