As a lifelong Catholic, Scalia never claimed to have an "exact moment" when he was "saved", therefore, by their own definition, he was "not a Christian"
I’m not questioning your honesty but in my ten years here I can’t recall a single time I’ve read such a statement. Are you sure you understood their point? We are discouraged from dragging arguments across threads but I can tell you that someone’s salvation is based upon their faith in Jesus Christ and not on which church they belong to. Only God sees the hearts of men and knows if faith is genuine. It’s not for me or you to proclaim who is or isn’t a Christian. I was born and raised a Catholic but didn’t become a real Christian until I was sixteen and heard the gospel in a Southern Baptist Church Sunday school. Jesus said we can know others by their fruit. Scalia bore good fruit by his life, words and actions. I have no doubt he was a Christian - Catholic or not - and that he is rejoicing in the presence of our Savior.
Catholics are saved at the moment of their Baptism. The Sacrament takes away all sin.