God’s kindness to us is the reason he made it possible for us to be saved from judgment if we simply trust in Jesus. AT: “God saved you by grace because of your faith in him” (See: rc://en/ta/man/translate/figs-activepassive)
The word “this” refers back to “by grace you have been saved by faith.”
“Grace is not based on what a person does, so that no one can boast and say that they earned it”
“In Christ Jesus” and similar expressions are metaphors that frequently occur in the New Testament letters. They express the strongest kind of relationship possible between Christ and those who believe in him.
“Walk in” here is an idiom for “obey.” “follow” or “do” (See: rc://en/ta/man/translate/figs-idiom)