1Jn 5:16 If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that.
1Jn 5:17 All wrongdoing is sin, but there is sin that does not lead to death.
That is not the phrase “mortal sin”.
Nor is it a list of said sins.
Got something else?
Now we are left with guessing games: Which sins are mortal and which are venial.