Set under authority (upo exousian tassomenov). It is not easy to render the exact force of these words. The sense of the present participle with the verb eijmi, I am, is very subtle. The words set under are commonly understood to mean placed in a subordinate position; but this would be more accurately expressed by the perfect participle, tetagmenov. The present participle indicates something operating daily, and the centurion is describing not his appointed position so much as his daily course of life. The word set originally means arranged, drawn up in order; so that the words might be paraphrased thus: "I am a man whose daily course of life and duty is appointed and arranged by superior authority." The centurion speaks in a figure which is well explained by Alford: "I know how to obey, being myself under authority; and I know how others obey, having soldiers under me. If then I, in my subordinate station of command, am obeyed, how much more thou, who art over all, and whom diseases serve as their Master." Just what estimate of Jesus these words imply we cannot say. It seems evident, at least, that the centurion regarded him as more than man. If that be so, it is a question whether the word man (anqrwpov) may not imply more than is commonly assigned to it. Taking the Greek words in their order they may read, "For I also, a man (as compared with thee), am set under authority, having soldiers under myself. See on Matt. viii. 9.

Robertson's NT Word Studies