Man is blind because of sin,
Revelation makes him sure;
Without that, who looks within,
Looks in vain, for all's obscure.
Nay, look closer into man!
Tell me, can you find indeed
Nothing sure, no moral plan
Clear prescribed, without your creed?
No, I nothing can perceive!
Without that, all's dark for men.
That, or nothing, I believe.--
For God's sake, believe it then!
This is in accordance with St. Thomas Aquinas's teaching on the natural moral law and the practical need for revelation. In his Summa Theologiae (IaIIae, q. 91, art. 4), St. Thomas states:
Besides the natural and the human law it was necessary for the directing of human conduct to have a Divine law. And this for four reasons, . . . [the second being] because, on account of the uncertainty of human judgment, especially on contingent and particular matters, different people form different judgments on human acts; whence also different and contrary laws result. In order, therefore, that man may know without any doubt what he ought to do and what he ought to avoid, it was necessary for man to be directed in his proper acts by a law given by God, for it is certain that such a law cannot err. [Ut ergo homo absque omni dubitatione scire possit quid ei sit agendum et quid vitandum, necessarium fuit ut in actibus propriis dirigeretur per legem divinitus datam, de qua constat quod non potest errare.]
*"pis aller" is a French word meaning worse (pis) and to go (aller). It means the final recourse or expedient, the last resort, last device, a dernier ressort, or stop gap measure.