I'm a Calvinist, and we believe that God's salvation comes to us purely by grace through faith (Ephesians 2:8,9). How is my belief unbiblical?
Where you push the boundary is in that doctrine of absolute determinism.
Determinism
The doctrine that all events, including human action, are ultimately determined
by causes regarded as external to the will. Some philosophers have taken determinism
to imply that individual human beings have no free will and cannot be held morally
responsible for their actions. (wiki)
That's what a Calvinist believes is the fundamental doctrine of Christianity.
God determines each person's destiny, any person's will is of no effect.