I have seen Roman Catholics use the 1 John 5.16-17 passage to justify the distinction between venial and mortal sins, which they take for grounds to suppose purgatory (a separate but related issue), but it is not exactly a foundational text for them to prove purgatory.
1 Cor. 3.13 is more commonly used by Catholics to support the doctrine of purgatory. In Matthew Poole's
Dialogue Between a Popish Priest and an English Protestant, p. 162, the popish priest relies on that verse heavily but is refuted by the protestant. I would encourage you to read that section on purgatory and the chapter on purgatory in Loraine Boettner's
Roman Catholicism to better understand where the Roman Catholic doctrine of purgatory comes from and then consult Matthew Henry's commentary, Matthew Poole's Annotations (John Howe wrote on 1 John), and John Calvin's commentary on 1 John 5:16-17 and 1 Cor. 3, and Calvin's
Institutes on purgatory/mortal v. venial sins as well.
Hope this helps a bit.
__________________
Andrew Myers
Husband of Jessica, Father of Jackson, Katie and Samuel
Member, Presbyterian Reformed Church of Northern Virginia
Warrenton, VA USA
Editor,
The Matthew Poole Project
"Let your Morning Thoughts, and your last Evening Thoughts, be what shall become of you to all Eternity." -- Matthew Poole