Faith, by definition, is, well, faith. How can we spurn something that we have faith in. The evidences isn't as it seems.
1Jo 2:19 They went out from us, but they were not of us; for if they had been of us, they would [no doubt] have continued with us: but [they went out], that they might be made manifest that they were not all of us.
I agree that not all of us are the some of us that the some of us are.
Yet we are exhorted throughout the Gospels to do. And we are informed that some will fall away who were previously believers. The only way to lose faith is to spurn that in which you have have previously had faith.