Be this your wall of brass — no secret sin,
To pale the cheek and rack the heart within!

[Hic murus aeneus esto,
nil conscire sibi, nulla pallescere culpa.]

Horace (65–8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Epistles [Epistularum, Letters], Book 1, ep. 1 “To Maecenas”, l. 60ff (1.1.60-61) (20 BC) [tr. Martin (1881)]
    (Source)

(Source (Latin)). Other translations:

Not to be giltye or war wan at anye falte at all,
A bulwarke that, to beare all bruntes, be that the brasen wall.
[tr. Drant (1567)]

Be this a wall of Brass, to have within
No black accuser, harbour no pale sin.
[tr. Fanshawe; ed. Brome (1666)]

Be this thy Guard, and this thy strong defence,
A vertuous Heart, and unstain'd Innocence;
Not to be conscious of a shameful sin:
Nor yet look pale for Scarlet Crimes within.
[tr. Creech (1684)]

True, conscious Honour is to feel no sin,
He ’s arm'd without that’s innocent within;
Be this thy Screen, and this thy Wall of Brass.
[tr. Pope (1737), ll. 93-95]

Be this thy brazen bulwark of defence,
Still to preserve thy conscious innocence,
Nor e'er turn pale with guilt.
[tr. Francis (1747)]

Be good, then, and be great;
This be your tower of strength, your throne of state;
To keep your heart unconscious of a sin,
And feel no goadings of remorse within!
[tr. Howes (1845)]

Let this be a [man’s] brazen wall, to be conscious of no ill, to turn pale with no guilt.
[tr. Smart/Buckley (1853)]

Be this your wall of brass, your coat of mail,
A guileless heart, a cheek no crime turns pale.
[tr. Conington (1874)]

Let this be a wall of brass around you -- "Not to be conscious of crime, or of any fault which spreads paleness over the countenance."
[tr. Elgood (1893)]

Be this our wall of bronze, to have no guilt at heart, no wrongdoing to turn us pale.
[tr. Fairclough (Loeb) (1926)]

And this bronze wall should be ours: to let no shame
Steal across our faces, no guilt steal into our hearts.
[tr. Palmer Bovie (1959)]

Make this your barrier of bronze,
that no crime burdens you, no guilt has turned you pale.
[tr. Fuchs (1977)]

Let a man stand
Behind this bronze wall:
Never guilty,
Never pale with sin, and fear
Of sin.
[tr. Raffel (1983)]

Let this be our defense: not to have any
Wrongdoing on our conscience to worry over.
[tr. Ferry (2001)]

So let this be your wall of brass:
to have nothing on your conscience, nothing to give you a guilty pallor.
[tr. Rudd (2005 ed.)]

Let that be your wall of bronze,
To be free of guilt, with no wrongs to cause you pallor.
[tr. Kline (2015)]