The entrance of thy words giveth light; it giveth understanding unto the simple.
The entrance of thy words giveth light - like the sun's beam entering through an aperture into a dark room. So "the Lord opened the heart of Lydia" to attend savingly to the Word; then the light gained an "entrance" into her soul. What then are we to think of those who deny the people of God free access to the Word of God? The Hebrew is literally 'the opening of thy Word' - i:e., the opening of the eyes of the soul caused by thy Word (entering). The Chaldaic somewhat similarly, 'The impression of thy Word.' The allusion is thus to the Urim (meaning light). What the Urim and Thummim were to the consulting worshipper, that thy Word is to me, giving me a response on all questions of faith and duty. So the Septuagint uses here the term [deeloosis] by which they ordinarily express the Urim. The Arabic, Septuagint, and Vulgate take 'the opening of thy Word' to mean, 'the opening of it by explanation' or 'manifestation.' So Hengstenberg, 'The explanation of the sense of the Word, imparted by God through His Spirit: to the natural man the doors of the Word of God are shut.' I prefer the former, which is much the same as the English version.
It giveth understanding unto the simple - (Psalms 19:7.) There is a play on similar sounds in the Hebrew between "entrance" and "simple" - peetach (H6608) ... pªtaayiym (H6612).