Because he says things like this:
The only noble sense in which we can claim to believe a thing is when we ourselves are living in the inner spirit of the thing.
I have no right to say I believe in God unless I order my life as under his all- Eye.
I have to right to say I believe that Jesus is the Son of God unless in my personal life I yield myself to that Eternal Spirit, free from all self-seeking, which became incarnate in Jesus.
I have no right to say that I believe in forgiveness as an attribute of God if in my own heart I cherish an unforgiving temper. The forgiveness of God is the test by which I myself am judged. (The Complete Works of Oswald Chambers, 385)