SIMPLE (Heb. pethî, silly, Gr. akakos, akeraios, harmless). The basic idea of the word in the OT is “easily influenced” (Ps.19.7; Ps.119.130; Prov.7.7). The two uses of the word in the KJV of the NT (Rom.16.18-Rom.16.19) are rendered “naïve” and “innocent” in the NIV.

International Standard Bible Encyclopedia (1915)

sim’-p’l: In the Old Testament the uniform tranlation of the Hebrew word pethi (root pathah, "be open"). Like the English word "simple" (etymologically "of one fold"), the Hebrew pethi is used sometimes in a good sense, i.e. "open-minded" (Ps 19:7; 116:6; 119:130, possibly in all three cases the sense is neutral rather than positively good), and sometimes in a bad sense (Pr 7:7, parallel to "destitute of understanding"; Pr 8:5, parallel to "fools" (blockheads); Pr 14:15, opposed to prudent). The fundamental idea of pethi seems to be open to influence, i.e. easily influenced. That one open to influence should as a rule be classed with the irreligious is one of many instances in which language is an unwilling witness to the miasmatic moral atmosphere in which we live. The line between moral weakness and moral turpitude, between negative goodness (if indeed such a thing be conceivable) and positive badness, is soon passed.

In the New Testament the word "simple" is found only in Ro 16:18,19 the King James Version. In the first of these passages it is used to translate akakos (the Revised Version (British and American) "innocent"). In Heb 7:26 the King James Version the same word is rendered "harmless," the rendering of the Revised Version (British and American) in this instance being "guileless." This would suit Ro 16:18 better than "innocent." Guilelessness is not a synonym for gullibility; but the guileless are frequently the prey of designing men. In Ro 16:19 the word translated "simple" is akeraios, literally, "unmixed," "sincere" (Trench and Godet; Young, erroneously "hornless" and so "harmless"). "Uncontaminated" seems to be the idea of the apostle. He would have those to whom he wrote "wise as regards good" and not ignorant as regards evil--for that would be impossible, even if desirable--but without that kind of knowledge of evil that comes from engaging in it, as we say, mixing themselves up with it, unalloyed with evil.