What is the biblical basis for saying God is a Subsistent Being?

Thanks to the new tag, 'aeseity', I've learned the definition for this, as representing God as absolutely independent and self-existent by nature. My problem now lies in seeing how 'subsistent' connects, for if Thomas Aquinas "explained the Triune Persons as subsistent relations", was he saying the three Persons are equally self-existent yet all are absolutely independent? I can't see that he was, for that would seem to be a contradiction in terms. However, you don't want philosophical points, but a theological view.

Perhaps the key to use to unlock this is to start with the orthodoxly Christian view that the one God is complex, not simple. The Welsh Reformed Protestant clergyman you quoted expresses that fact beautifully. I consulted the writing of Harold O.J. Brown, remembering he had spoken about 'subsisting' early on in his monumental history of doctrines and heresies in the Church.

In the early 200s A.D., the Trinity was being debated, and language framed to bring clarity. Before the word 'subsits' was used to express the mystery of God in three Persons, thinkers like Origen explained how the words 'Father' and 'Son' referred to a relationship that is eternal, not to an act in time (meaning the Son was always with the Father, an eternal 'begetting'). Origen's imagination was so fertile and his speculation so elaborate that the Eastern Orthodox regard him as a heretic, and great liberals, such as Harnack, embrace him (see page 86 of Brown's book, below). However, Origen's points paved the way for grasping the eternal nature of that relationship within the Godhead. Now here is a quote dealing with the question:

"Later theological language will express the mystery of the Trinity by saying that God subsists in three Persons... By introducing a rare term, 'subsists', theology seeks to show that the Trinity is not to be demystified by resorting to inapplicable comparisons. If one were to say, for example, that God consists of the Father, the Son, and the Holy Spirit, this might appear to suggest that God has three parts. By teaching that the Father eternally begets the Son, and that the Son, eternally begotten before all time, is thus effectively co-eternal with the Father, Origen prepared the way for our present understanding: God does not consist of parts, but subsists in Persons. These Persons are distinguished from one another by means of a relationship - in the case of the Father and the Son, by begetting and being begotten - but not by succession in time." (Heresies and Orthodoxy in the History of the Church, p89, Harold O.J. Brown, Hendrickson & Baker Book House Co., 1998)

The point is then made that such theology is not pedantic or meaningless today because such precise language was, and still is, needed to avoid falling prey to misunderstanding, if not actual heresy. Regarding Origen and the doctrine of the trinity, the author adds that it:

"will always remain a mystery, but thanks to Origen, it can at least be expressed in such a way that we can see wherein the mystery lies." (Ibid. p.91)

There is a tendency for those who like to give the impression that they can explain God, to play fast and loose with theology about their comparatively simple God, who can be understood, in what are usually simplistic terms. That charge cannot be leveled at the words used to detail the Trinity doctrine, because the Godhead is complex, not simple. Certainly, words like 'subsists' and 'aseity' are difficult words, and are not found in the Bible, yet the concepts surely are, as those quotes from Martyn Lloyd-Jones show.

You ask for the biblical basis for saying God is a subsistent being, and I see the many scriptures used by Dr. Martyn Lloyd-Jones as providing that basis. Yet there are very many more Bible texts that could be listed to show the eternality of the three Persons, and their utter unitedness as the one God, for example, John 5:26, 4:14; Gal.6:8; Psalm 83:18 in conjunction with Isaiah 63:10 and Exodus 17:6; Hebrews 9:14; 1 Cor.2:10-11; Psalm 139:7-10; 1 Cor.3:16 in conjunction with 2 Cor.6:16; Romans 5:2 in conjunction with Ephesians 2:18; Isaiah 40:13-14 in conjunction with 48:16. (Note now many of those verses deal with the Holy Spirit as God.)