Letter from Waller to Hopton 1643
Jun 16 To my noble friend Sir Ralph Hopton at Wells.
Sir

The experience I have had of your worth, and the happiness I have enjoyed in your friendship are wounding considerations when I look upon this present distance between us. Certainly my affections to you are so unchangeable, that hostility itself cannot violate my friendship to your person, but I must be true to the cause wherein I serve. The old limitation 'usque ad aras' holds still, and where my conscience is interested, all other obligations are swallowed up.

'usque ad aras' means 'even to the altar' ie 'for ever'.
I should gladly wait on you according to your desire but that I look upon you as you are engaged in that party, beyond a possibility of retreat and consequently uncapable of being wrought upon by any persuasion. And I know the conference could never be so close between us, but that it would take wind and receive a construction to my dishonour.
That great God, which is the searcher of my heart, knows with what a sad sense I go upon this service, and with what a perfect hatred I detest this war without any enemy, but I look upon it as Opus Domini, which is enough to silence all passion in me. The God of peace in his good time send us peace, and in the mean time fit us to receive it. We are both upon the stage and must act those parts assigned to us in this tragedy. Let us do it in a way of honour, and without personal animosities, whatsoever the issue be, I shall never willingly relinquish the dear title of

Your most affectionate friend and faithful servant,
William Waller.

'Opus Domini' - Work of God