John Gill on "The Immutability of God's Decrees"

God is unchangeable in his purposes and decrees, there is a purpose for everything, and a time for that purpose; God has determined all that ever was, is, or shall be; all things come to pass according to the counsel of his will, and all his decrees are unchangeable; they are like the laws of the Medes and Persians, and more unalterable than they were; they are the mountains of brass Zechariah saw in a vision, from whence proceed the providences of God, and the executioners of them (Zech. 6:1), called “mountains” because of their immoveableness, and mountains of “brass” to denote their greater firmness and stability: immutability is expressly spoken of the counsel of God (Heb. 6:17), the purposes of God are always carried into execution, they are never frustrated; it is not in the power of men and devils to disannul them; whatever devices and counter workings to them may be framed and formed, they are of no avail; “the counsel of the Lord stands for ever” (Ps. 33:11; Prov. 19:21, 21:30; Isa. 14:24, 27, 46:10), the purposes of God are “within” himself (Eph. 1:9), and what is in himself, is himself, and he can as soon cease to be as to alter his mind, or change his counsels; and they are “eternal” (Eph. 3:11) no new thoughts arise in his mind, no new resolutions are formed in his breast, no new decrees are made by him; his counsels are “of old”; and his purposes are called “counsels”, because designs wisely formed by men, are with consultation, and upon mature deliberation: and such are the decrees of God, they are made with the highest wisdom by him, who is wonderful in counsel, and excellent in working, and so are unchangeable: and besides, being “all-knowing”, he sees and declares the end from the beginning, and nothing unforeseen ever can appear to hinder the execution of his intentions and determinations; which is sometimes the case with men: and he is “able” to perform whatever he resolves upon; there is no lack of wisdom, nor of power in him, as often is in men; and he is “faithful” to himself, his purposes and decrees; his “counsels of old are faithfulness and truth”; or are truly and faithfully performed.