Mortal sin is mortal sin. We are supposed to grow in holiness, not to feel comfortable in our ignorance.
Ignorance and deliberate consent are diminishing factors after the question as to whether negligence exists.
The question of due diligence in knowing what the Church teaches often means that a person is voluntarily responsible for their actions. I’ll admit, there are cases (not some) where reasonable due diligence is impossible, but they are not that often though in a society with the access to information we have.
One cannot simply reject the teaching of the Church because you don’t understand it, and expect for that to be a diminishing factor. The law is difficult because it is mean to teach us, not to engage in legalism. The Church has authority to teach us because it comes from God who we must obey, not just because, at that time, we think it is a great idea.
This is because, understanding comes to us often after doing a thing. Children have to be taught this way. And we are children to God.
The simple Biblical truth is:
“Just as you used to offer yourselves as slaves to impurity and to ever-increasing wickedness, so now offer yourselves as slaves to righteousness leading to holiness.
When you were slaves to sin, you were free from the control of righteousness.
What benefit did you reap at that time from the things you are now ashamed of? Those things result in death!
But now that you have been set free from sin and have become slaves of God, the benefit you reap leads to holiness, and the result is eternal life.
For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.”
The point of the above passage is that we are all condemned sinners, but God took the initiative to provide a way of escape and salvation. To be free to live for Him and serve Him.
Becoming a Christian means admitting that we are helpless sinners, that Jesus is God’s Son, that He died for our sins and rose to life again. It means allowing Christ to be the Lord, Boss, the CEO of our lives - not us - Him.
Living a life of activity that is blatantly disobedient to God’s teaching for moral living is evidence that that person has never really allowed Christ to be Lord of their life, and thus, are not saved. They are either hypocrites or they’ve just fooled themselves into thinking that, somehow in their case, what they do is justified.
Only God knows the heart of people. I do know that we all, even as Christians, will continue to sin - but, it should with less and less frequency. A Christian’s life should take on, over time, more and more of the character of Jesus.
On the other hand, someone who lives a continual life of disobedience to God’s moral teachings, who shows no change in their behavior, I think that person is probably not really a Christian. But, it is God who decides than (thankfully), not me or us.