This is actually an excerpt from a discussion I had with Bob Wilson a few years ago (late November, 2008) here on the forum, where I laid out two formal arguments linking universalism with orthodox trinitarian theology.
I got tired of trying to hunt them up whenever I wanted to reference them, so I’m copying them over to this new thread for ease of reference.
The first argument attempts to show how I arrived at (some kind of) universalism as a deductively logical corollary from ortho-trin theology. i.e., as a matter of my personal history, it was working out the logical corollaries of ortho-trin in this fashion which first landed me on believing universalism to be true rather than just sorta-vaguely-hoping-somehow-it-might-be-true. Before then I believed I ought to still affirm some kind of hopeless final condemnation (despite some significant scriptural testimony apparently otherwise!–since after all there’s also some significant scriptural testimony apparently in favor of hopeless condemnation, too), even if I wasn’t sure exactly what kind I ought to affirm.
P(roposition)1.) Supernaturalistic theism is true. (Setting aside how one gets to this proposition, which would require prior metaphysical analysis.)
P2.) Theism involves intentional behavior at (and as) the foundation of all reality; distinct from atheism which involves either no behavior or only unintentional behavior at (and as) the foundation of all reality. (Also requires prior analysis.)
P3.) God is a self-begetting, self-begotten interpersonal unity. I.e. at least binitarian theism is true. (Should be established on prior analysis. Note that because the entity is singular and personal, singular-personal terms can be used in reference to the entity, which when being spoken of corporately should be considered singular.)
C(onclusion)1.) God’s self-begetting and self-begotten-ness involves intentional behavior at (and as) the foundation of all reality. (from P2, P3)
C2.) God’s self-begetting and self-begotten-ness involves intentional behavior at the foundation of His reality, too. (from P1 (if true, God is real), C1.) i.e., what philosophers call “positive aseity” is true: God depends upon His own action for self-existence.
P4.) Not-God entities exist. (Exclusively implied by reaching P1; i.e. “naturalistic theism” aka pantheism isn’t true.)
P5.) Some not-God entities (included within P4) are also persons. (Should be established, insofar as possible, on prior analysis. Incidentally, I would analyze this proposition and counter-propositions before arriving at P1 etc. One result is the discovery that this can only be self-reflexively presumed, not proven; although hypothetical counter-presumptions can be analyzed and rejected as incompatible with claims tacitly made in argumentation.)
C3.) All not-God persons depend upon God’s intentional self-begetting and self-begotten behavior for existence, including as persons. (from C1, P5.)
C4.) God’s own self-existence depends on the Persons of God acting to fulfill cooperative interpersonal unity. (From P3 (God exists as an interpersonal unity), C2.)
C5.) God would cease to exist if any or all of the Persons acted against fulfilling cooperative interpersonal unity. (implied by C4.)
O(bservation)1.) I exist.
C6.) I can be sure God (in any Person or corporately) will never act against fulfilling cooperative interpersonal unity between persons. (from C3, C4, C5, O1.)
O2.) I sometimes act against fulfilling cooperative interpersonal unity between persons (i.e. against fair-togetherness, i.e. “unrighteously”).
C7.) I sometimes act against the foundation of all reality, including my own. (from O2, C3.)
C8.) My action against the foundation of all reality, including my own, leads to my cessation of existence, without intervention from God. (from C3, C5 (implied subordinate parallel), C7.)
C9.) God intervenes to save me, the doer of non-fair-togetherness, from cessation of existence (from O1, C7, C8.)
H(ypothesis)1.) God eventually chooses to allow me to cease to exist (at least as a person), or acts to take me (at least as a person) out of existence. (Note that this covers annihilationistic versions of Calvinistic soteriology as well as Arminianistic soteriology, practically speaking; the difference between the two being God’s original intention regarding the ultimate salvation of the sinner from sin.)
H2.) God keeps me in existence as a person; but either refuses to ever act to lead me to fair-togetherness, or else gives up acting toward this end. (Note that this covers non-annihilationistic versions of Calvinistic and Arminianistic soteriology. Note that “Calvinistic” and “Arminianistic” are meant to broadly cover principly similar doctrinal sets of salvation and condemnation in groups or denominations not historically Calvinist or Arminian per se.)
H3.) God keeps me in existence as a person; and persistently acts to lead me to fair-togetherness with other persons (including with Himself).
O3.) If H1, then contra C6.
O4.) If H2, then contra C6.
O5.) If H3, then no contradictions through C9.
C10.) If set through C9 is true, and if set H1-3 is exhaustive as a response from God to my sin, then H3 must be true.
H3 is perhaps the most minimal definition of theistic universalism. The argument as provided doesn’t specifically attempt to go further with inferring more particular details about the doctrine. (e.g. is there a purgatory?–is that hell?–is ultra-universalism true instead?–what, if anything, is the responsibility of the sinner in this?–can we be sure God will succeed at this? etc.)