Yahwism is the religion practiced by the Hebrews and Israel during the pre-captivity Old Testament. Yahwists worshipped the one true God who calls himself Yahweh. After God came to Earth in incarnate form, Christianity was founded and succeeded the religion. While Yahwism has fundamentally the same doctrine as Christianity since both only are faith based, the two are considered separate for chronological reasons. The term Yahwism is used in order to distinguish the worship of the one true God from the practice of polytheistic religions.
Yahwism began shortly after the expulsion from the Garden of Eden, as originally Adam and Eve could directly interact with God (similar to Heaven) and therefore did not need a religion or "faith". After they were expelled, further relationship with God required faith. While Yahwism always required faith, with Moses it eventually introduced regulatory laws to the nation of Israel (documented in Pentateuch primarily in Leviticus), requiring sacrifices for the atonement of sin.
Several thousand years later the religion of Judaism was founded, with additional regulatory requirements in order to fulfill the God given ones. Christianity eventually succeeded, and replaced Yahwism, as Jesus Christ became the permanent atonement for sins.
Originally in the Garden of Eden, faith through religion was not required. Adam and Eve had a direct relationship with God physically present in the Garden. The two could literally hear God's voice and movement through the Garden so they did not need "faith", as that is is when what cannot be seen.
The Name Yahweh
Prior to Moses