Trinitarianism makes a lot of self-contradictory claims: on the one hand, trinitarians typically affirm that the one God is immortal, immutable, invisible, un-temptable, all-knowing, etc; but at the same time, they propose that this one God, in his second person, died, changed, was seen, was tempted, and didn’t know certain things. This is an obvious logical contradiction- for the same subject to be said to be un-killable yet be killed, un-temptable yet be tempted, etc, is just obviously a contradiction and a falsehood; and so, this simple line of reasoning about the attributes of God in contrast to the attributes we see Christ had is frequently used to show that the doctrine of the trinity is false. Jesus Christ cannot be the one God if his attributes differ so widely from those of the one God; the difference makes it obvious that the one God is one, and Jesus Christ his human Son is another.
But trinitarian apologists have been working hard to try to answer this, ever since the doctrine of the trinity was invented. It’s an obvious problem for them; and solution has typically been to try to find some way that God can be both immortal, and have died, be immutable, yet have changed, etc. The solution is typically sought through incarnation theories- something about the second nature the second person of the one God assumed allows these things, it’s conjectured. Other theories abound as well though, such as proposing that due to divine timelessness, God can be eternally be two opposite things, like mortal and immortal, because technically He wouldn’t be these things at the same time, thus arguably escaping the charge of being a contradiction.
But here is a serious problem with all these explanations: this trinitarian reasoning used to try to save the doctrine of the trinity from logic is a terribly slippery slope; any trinitarian argument that says that although God is immortal He can die, will generally also lead to the same logic for all God’s attributes; allowing that He can be both evil and good, all knowing and ignorant, unchanging and changing. After all, if God being incapable of dying doesn’t actually mean he cannot die, then on what basis can we say that God being incapable of sinning means that He will never actually sin? If God’s immortality is the kind of immortality where one can still die, and God’s immutability is the kind of immutability where one can still change (which, of course, is in truth no real immortality or immutability at all), then how do we know that God’s goodness is not the kind of goodness where one can still commit evil? How do we know that God’s holiness is not the sort of holiness which allows one to be defiled? How do we know that God’s perfection isn’t the sort of perfection that allows flaws and errors?
Once you reach immutability with this reasoning, it all implodes though. If God can change, that is, in His very nature and character, then revealed religion is worthless, as we have no idea if God will even be good tomorrow. If one responds that He is trustworthy, we may respond that if He can change then He might well not be trustworthy tomorrow. If He can change then He might be unfaithful to His promises. Thus scripture assures us, in the context of those promises, that God does not change, so that we may rest assured in them (Mal 3:6). But if God may be the opposite of anything He is, then we cannot make any certain assertion about God, and what is true of Him now could be false the next minute. In short, these trinitarian arguments would prove too much: they would be an argument for deism rather than the trinity.
And so then, the trinitarian defense of their contradictions is a slippery slope: if God can be temptable although untemptable, mortal while immortal, change while unchangable, etc, then these statements about Him mean nothing, and it would reasonably be just as possible that while being good He might be evil. This trinitarian logic taken to it’s logical ends, if true (which it isn’t) would destroy Christianity and all revealed religion. Our ability to positively assert truth about God would be lost entirely, for anything we say He is, He might in fact turn out to be the opposite; we will have lost out ability to speak meaningfully about God at all.
Of course, sometimes trinitarians seem to affirm this point already- they like to point to the supposed insufficiency of human language to speak accurately about God, whenever they find human language making things to concrete for them to sip their contradictions through unnoticed- which is fairly often. But this point is as slippery of a slope as the one noted above; if our human words are truly incapable of accurately relaying truth about God, then God’s endeavor to reveal Himself to us through human words, in the scriptures, and in the oral teachings of the prophets, Christ, and the apostles, has failed. I reiterate again: if human words cannot accurately communicate truth about God, then God has failed, because this is precisely what God has set about doing. If we believe that God is too wise and too powerful to fail, and trust that He knows what He is doing a great deal better than we do, then we will rather need to accept that revelation- which always comes to us in human language- as an accurate and truthful way of communicating truth about God. As God is ultimately the Maker of man and of human language, we ought not be surprised that He has allowed and designed things such that it is capable of communicating truth about Him.
The alternative to this is deism; if we believe in God, the Supreme Being, but deny His ability to accurately reveal Himself in the main way that He has set about doing so, viz, through human language, then we will be forced to be totally agnostic about God. Our reading that He is good will not mean much, when by now, for all we know, He has already taken on another nature that is evil, and so is now both good and evil. Or, for all we know, He took on an imperfect nature alongside His perfect one, and is now as flawed as we are. These trinitarian defenses, then which all depend on proving that God can actually in some way be the opposite of the way He is, don’t actually help the doctrine of the trinity at all, because the logical end of this reasoning is to deny the validity of special revelation about God altogether. This is something trinitarians need to take to heart; this is a clear reductio ad absurdum for most or all trinitarian attempts to justify how an immortal person can die, and an un-tempable God can be tempted by evil, etc. Yet without these sorts of arguments, trinitarians are left with logical contradictions that are as serious as they are numerous, all of which work to show us that the doctrine of a triune God is false. If we are really committed to the reality of meaningful and accurate divine revelation from God through words, as we have in the scriptures, then we will need to find a better explanation for the biblical data we have been given than what the doctrine of the trinity can provide us with.