Thomas Aquinas

The path of the just is like a shining light, et cetera. Prov. IV, 18. In this authority the life of the saints is described from three things. From their narrowness, there, path, because Matth. VII, 14: narrow is the way, etc.; Job XXVIII, 7: the path the bird has not known, et cetera. From their brightness, there, a shining light. Eph. V, 8: you were once darkness, et cetera. For the just are shining, and therefore their way is bright. From their advancement, because it always grows. I Peter II, 2: that in it you may grow, et cetera. And this even unto the perfect day, namely of glory. I Cor. XIII, 10: when that which is perfect has come, it shall be done away, et cetera. Conversely the way of the wicked is broad; dark, shadowy, and failing. Whence Prov. IV, 19 is added the way of the wicked is dark, et cetera. And Matth. VII, 13: wide is the gate, and broad is the way, that leads to destruction, et cetera. From these words the matter of this epistle can be drawn. For the Philippians were in the right path of Christ, sustaining many tribulations for Christ. Likewise, illumined through faith, below II, 15: among whom you shine, et cetera. Likewise, they were advancing, as is clear through the whole epistle. Likewise, fittingly after the epistle to the Ephesians, in which instruction is given as to how ecclesiastical unity is to be preserved, these are set forth as an example of preserving ecclesiastical unity, who preserved it best.