Translated by Ch. Browne and J. Swallow. Cf. An Introduction to the Theological Orations of St Gregory
III. Not to every one, my friends, does it belong to philosophize about God; not to every one; the Subject is not so cheap and low; and I will add, not before every audience, nor at all times, nor on all points; but on certain occasions, and before certain persons, and within certain limits.
Not to all men, because it is permitted only to those who have been examined, and are passed masters in meditation, and who have been previously purified in soul and body, or at the very least are being purified. For the impure to touch the pure is, we may safely say, not safe, just as it is unsafe to fix weak eyes upon the sun's rays. And what is the permitted occasion? It is when we are free from all external defilement or disturbance, and when that which rules within us is not confused with vexatious or erring images; like persons mixing up good writing with bad, or filth with the sweet odours of unguents. For it is necessary to be truly at leisure to know God; and when we can get a convenient season, to discern the straight road of the things divine. And who are the permitted persons? They to whom the subject is of real concern, and not they who make it a matter of pleasant gossip, like any other thing, after the races, or the theatre, or a concert, or a dinner, or still lower employments. To such men as these, idle jests and pretty contradictions about these subjects are a part of their amusement.
IV. Next, on what subjects and to what extent may we philosophize? On matters within our reach, and to such an extent as the mental power and grasp of our audience may extend. No further, lest, as excessively loud sounds injure the hearing, or excess of food the body, or, if you will, as excessive burdens beyond the strength injure those who bear them, or excessive rains the earth; so these too, being pressed down and overweighted by the stiffness, if I may use the expression, of the arguments should suffer loss even in respect of the strength they originally possessed. 
 i.e. Should not only fail to be strengthened thereby, but be actually weakened, through their inability to understand the argument. A bad defence weakens a good cause.
Reference address : https://www.elpenor.org/gregory-nazianzen/against-eunomians.asp?pg=2