Stu: Catholics believe that Priests can forgive their sins.
Many, but not ALL Catholics believe this.
Stu: YOU need to pray to God and ask for forgiveness in Jesus name.
We do need to ask God for forgiveness. Constantly so. But ALL Christians, positionally, are ALREADY forgiven of all past, present and future sins - even ones we are unaware of. So, it's not like one dies suddenly, and didn't get a chance to say that last prayer of repentance of some sin.
Some Catholics pray to Mary and the saints.
Some certainly do - actually, many of them do. The practice is unBiblical, without any NT examples of it. But it is also unnecessary, as God hears ALL prayers, and Jesus is our Great High Priest. The other thing this does is that it often takes the focus off of God, as it focuses upon saints now in Heaven. That also can take away from relationship with God. It is understandable how this occurs - not only due to Catholic teaching that can encourage it, but also because these saints were humans, and so people can sometimes more readily identify with another human who is a supposed intercessor - that somehow makes our beseeching more effective. Also, what people fail to realize is that pray is far more learning to align our wills and wishes with those of God. And do people not realize that God has ALWAYS known what ALL of their prayers would be - all they will ever pray in their entire life? Do they not also realize that God also has ALWAYS known of their circumstances before they pray, of which prayers and how He will or will not answer them? God is not deficient in which He needs some intermediary to hear the prayers of those who beseech Him. He needs no middle man or woman, "dead" or alive.
Stu: Mary does not hear your prayers, she is dead.
That is not a certainty. Those Christians who die are instantly in the presence of the Lord (Apostle Paul). They are alive and know what is going on THERE, to whatever extent. What they know about us here is undertermined. At the least, they know which of their loved ones are not yet there, many of whom are Christians, many not. If prayer is a dialogue with God, why would people in Heaven cease such a dialogue? Would they not pray for those still here? Why not? HOWEVER, that is a very different thing than asserting A) that the saints in Heaven definitely hear our prays on earth, or B) that we are to direct any prayers to anyone other than the very one who has the power to answer them: GOD!!!
Stu: The RCC teaches that you need the RCC as a mediator between God and man. Which is total crapola.
I would say, at the very least, the RCC encourages it with man-made doctrine that is not found in the canon completed by the end of the first century. Catholics would deny that doctrine stopped there, as they believe that some office of Pope exists - it does not. Note also that miracles ceased with the Apostles. Ever know of Popes doing certified miracles??? Like the apostles did???!!!
Stu: There's many more things that are simply man-made rules made up by Pope's on a whim during each Pope's reign. For instance in 1229 is was ruled that laypeople are not allowed to read the Bible!!
At one time or another, they've all created new doctrines that contradict Scripture. They often done and ruled in despicable ways - and did so as official pronouncement of office. But don't believe me, delve in to the records of history.
But as for one's salvation, of course many Catholics are Christians, just as many Protestants are not (but THINK they are).