Institutes of the Christian Religion (1559) - John Calvin
Book 2 - God the Redeemer
Chapter 8 - Explanation of the Moral Law:
The 10 Commandments
Sections 58-59 - Law - mortal and venial sins distinction invalid!
Venial sin, they say, is a desire without intent, that doesn't stay long.
But it gets a foothold at all by our sin.
"There is no reason, then, for us to exempt any covetings, however light, from the judgment of death" - Rom 6:23.
Section 59 - Every sin a deadly sin!
Matt 5:19; Ezek 18:4, 20
...because it is committed against God.
If they persist in this error, "bid them farewell."
And hold that "all sin is mortal."