People believed in the ability of the man who crossed Niagara falls on a tight rope pushing a wheelbarrow, but they didn't trust enough to actually get in the wheelbarrow themselves.

Jesus Christ died on a cross for my sins, and your sins. Being a Christian is placing our trust in the work of the cross (the judgement he has already taken upon himself for us). It is putting our self in his hands, relying upon his mercy. Not trying to make it to God in our own power, but really trusting Jesus.