Suppose the monetary authority says "We already have interest rates at zero, so monetary policy can do no more to increase aggregate demand. But fiscal policy can do more..."
Suppose the fiscal authority says "We already have very high deficits and debt, so we are scared of future insolvency, so fiscal policy can do no more to increase aggregate demand. But monetary policy can do more..."
No one individual admits to a loss of faith that monetary or fiscal policy can increase aggregate demand as much as is needed. But policy is the same as it would be if there were such a loss of faith.