"If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death."
When human beings enter into noetic prayer in a state of theosis or complete union with God, then whatever they wish is what God wishes, and it is given them.
Matt 21:21: "Jesus replied, "I tell you the truth, if you have faith and do not doubt, not only can you do what was done to the fig tree, but also you can say to this mountain, 'Go, throw yourself into the sea,' and it will be done."
When the Son of Man returns, will he find such faith on earth?
St. John is speaking specifically of prayer which is both asked and answered, prayer which is in union with God, which is true prayer: "he will ask, and He will give.
One may certainly ask things apart from union with God or unison with his will, but is seems such asking is not true prayer in the sense St. John is speaking of, i.e. prayer which is both asked and answered ("he will ask, and He will give"); by such noetic prayer one becomes a vessel of the Holy Spirit; it is always efficacious. One may certainly ask for anything and call it prayer -this, I suspect, is a different matter.
Our saints have taught we can and should pray even for the demons; whether they will continue in the only mortal sin is unknown to all but God, but we cannot pray that one will be forgiven the unforgivable sin.
"There is a sin leading to death. I do not say he should pray about that."
The only sin that cannot be forgiven is refusal to repent, which in effect is a spurning of forgiveness.