John Calvin Commentary 1 John 5:15

"and if we know that he heareth us whatsoever we ask, we know that we have the petitions which we have asked of him." — 1 John 5:15 (ASV)

And if we know — this is not a superfluous repetition, as it might seem; for what the Apostle declared in general regarding the success of prayer, he now affirms specifically that the godly pray for, or ask for, nothing from God except what they obtain. But when he says that all the petitions of the faithful are heard, he speaks of petitions that are right, humble, and consistent with the rule of obedience. For the faithful do not give loose rein to their desires, nor indulge in whatever pleases them, but in their prayers always consider what God commands.

This, then, is an application of the general doctrine for the particular and personal benefit of everyone, so that the faithful will not doubt that God is propitious to the prayers of each individual. Consequently, with peaceful minds, they may wait until the Lord performs what they pray for. Being thus relieved from all trouble and anxiety, they may then cast on God the burden of their cares.

This ease and security, however, should not lessen their earnestness in prayer, for the one who is certain of a positive outcome should not refrain from praying to God. For the certainty of faith by no means generates indifference or sloth. The Apostle meant that everyone should be at peace in these necessities, once they have entrusted their sighs to the bosom of God.