Will someone list all the “mortal” sins and provide me with the divine source? To whom and when did God personally give this list?
If you google “examination of conscience” you will get some great lists. All are based on the ten commandments.
So then, (1) ultimately, you’re saved (or at least preserve your salvation) by keeping the Law, and (2) if you work on Saturday you lost your salvation. And remember, if you break one you break them all.
No, Catholics have a more mature understanding of salvation than this. God gave us commandments to help us live the way He designed us to live. They are a gift. Without grace, it is not possible to appreciate the gift of the commandments.
Phil 3:12-16
Not that I have already obtained this or have already reached the goal; but I press on to make it my own, because Christ Jesus has made me his own. 13 Beloved, I do not consider that I have made it my own; but this one thing I do: forgetting what lies behind and straining forward to what lies ahead, 14 I press on toward the goal for the prize of the heavenly call of God in Christ Jesus. 15
Let those of us then who are mature be of the same mind; and if you think differently about anything, this too God will reveal to you. 16 Only let us hold fast to what we have attained.
Catholics have received and follow the Apostolic Teaching that obedience is part of God’s work in us.
“Therefore, my beloved, just as you have always obeyed me, not only in my presence, but much more now in my absence, work out your own salvation with fear and trembling; 13 for it is God who is at work in you, enabling you both to will and to work for his good pleasure.” Phil 2:12-13
People are not able to obtain salvation of their own effort. We are saved by grace, through faith. The grace that saves is the grace that produces obedience and good works in us. These are not separated from one another, and they are all accessed through faith.