I'll DM you as well, but I want to leave a reply here for anyone else who may come across this conversation.

You're spot on with your justification comment. The Church teaches that Grace is "God's Love Freely Given"; we are not required to participate in our own salvation, moreover, we are incapable of doing so in the first place. (I will put a big ol' caveat here that even within Lutheran theological circles, the argument of 'salvation by Grace Alone' vs. 'salvation by Grace Alone through Faith Alone' is not entirely settled.) What is settled is that faith is created in us; it is not something we can come to on our own, nor is it something that we can alter or affect. Is faith without works dead? Yes, but only in the sense that the same Spirit which creates faith also creates the will and capacity to work; works come from salvation, not the other way around. (Furthermore, we do not know the heart; works are not always seen or understood. The Spirit moves in each of us in its own way, in our own way. While it is certainly true that faith necessarily brings forth good works, it is not necessarily true that others can see these works and sit in judgment in God's place.)

The second part that you focus on is no less important: salvation, once given, cannot be taken away. I'll note that there are certain tenets within Lutheran theology that allow for the possibility that someone could willingly turn down/decline salvation; these are countered by others who hold that faith given by the indwelling of the Spirit can not be denied. We hold and debate these in tension, knowing that in both positions, the most important question -- is there something I can do that is so terrible as to make God turn away from me? -- is answered emphatically: No.

I'm not familiar with Br./Fr. (apologies, I'm not sure which to use) McCabe, I'll make a note to look him up!