Suppose, you knew someone in real life and you knew them to be godly and spirit-filled, and constantly in prayer. And that person dies.
Even though we are not to assume anyone is in heaven, and only God knows the heart, what if you are reasonably sure they are in heaven? Is it wrong to pray for their intercession?
No, it isn’t. As a matter of fact, the road to canonization generally begins when a person on Earth prays for the intercession of a pious deceased person. Think about it. Someone must have prayed to Pope John Paul II before he was beatified. Someone must have prayed to St. Therese of Lisieux before she was beatified and ultimately canonized. So we find that canonization really begins when a person on Earth asks for the intercession of the faithfully departed.
So how do they become canonized? Well, if a medical miracle occurs because of the intercession of the pious deceased person, the case is taken up to the Congregation of Saints. They analyze, ask questions, try and rule out natural explanations, and the process continues and begins there.
That said, it’s important to note that you
do not need to be canonized to be a saint. A saint is merely a person who is in Heaven enjoying the Beatific Vision. Canonized saints are simply saints who have been recognized by the Church. Canonized saints make up a very, very small fraction of the number of saints in Heaven. Most saints in Heaven are unrecognized officially.
It isn’t wrong to pray for the intercession of a pious person who is deceased. But you have to take some degree of caution. You should only ask for the intercession of a pious Catholic. You should also make sure that they lived relatively holy lives. I say such because this increases the likelihood of the given person being in Heaven (it’s riskier with Protestants, Jews, Muslims, etc). If not in Heaven, they would be in Purgatory. Whilst it has not been put to print so to speak, it is in the “mind of the Church” that the Holy Souls in Purgatory can intercede on behalf of those on Earth.
Hope that helps.