John 4:24 God is Spirit, and those who worship Him must worship in spirit and truth.
If we were to follow the Old Testament only or the New Testament only, we would miss out on so much of the truth and how God worked with the people in Spirit as well.
Worshipping God with sacrifices because of laws broken will allow us to get to know God and obey Him. But to know Him properly we have to get to know Him in spirit as well.
Trusting the unseen God to work with us in ways we cannot understand is something many people find very hard to accept simply because we cannot see Christ in work physically.
Trusting in all three persons of the Trinity allows us to grow in strength with our God in many more ways than we thought possible to start with!