The Heidelberg Catechism was written in 1563. This particular question/answer explains the Christian viewpoint of salvation being my faith alone.

How are you right with God?
Only by true faith in Jesus Christ
Even though my conscience accuses me
of having grievously sinned against all God’s commandments
and of never having kept any of them,
and even though I am still inclined toward all evil
without my deserving it at all
out of sheer grace
God grants and credits to me
the perfect satisfaction, righteousness, and holiness of Christ
as if I had never sinned nor been a sinner
as if I had been perfectly obedient
as Christ was obedient to me
All I need to do
is to accept this gift of God with a believing heart.