A being cannot be both just and merciful. They are contradictions. Mercy is the suspension of justice.So I'll be clear and say that I assume that what is specifically being attacked here is the Christian view that God demonstrates both mercy and justice in His dealings with people. That is, those that are saved by grace through faith and redeemed by the the blood of Christ are shown mercy (thereby avoiding the just consequences of their sins) being allowed to enter into Heaven upon death, remaining there for eternity. In contrast, those that are not saved from their rebellious life of sinfullness are, upon death, banished to an eternal existence in Hell, thus being given just punishment for their life of disobedience and moral lawbreaking.
Allow me to frame my response by being explicit and grabbing the definitions for the two key words here from Merriam Webster. I'll choose the ones that I think fit the contextual useage best.
justice - 1a. the maintenance or administration of what is just especially by the impartial adjustment of conflicting claims or the assignment of merited rewards or punishments
mercy - 1a. compassion or forbearance shown especially to an offender or to one subject to one's power; also : lenient or compassionate treatment
Another definition worth making note of (because I'll mention it later) is that of the Law of Non-Contradiction - It is not possible that something be both true and not true at the same time and in the same context.
Before going farther, I will agree to the accuracy and correctness of the commenter's abridged definition of the two terms - namely that "Mercy is the suspension of justice".
That being said, I submit that the succint conclusion - that "A being cannot be both just and merciful." - is at worst, false, and at best, ambiguous (and therefore it's not possible to deal with it clearly).
First, let's look at how this statement is obviously false (in it's current ambiguous form). For example, I am a father, I have three children. I can think of numerous times in the course of their lives where they've done something that they should justly be punished for, and then I punished them. I can also think of numerous times in the course of their lives where they've done something that they should justly be punished for, but for my own reasons at that particular time, I chose to not punish them, but rather show them mercy and simply discuss with them why what they did was wrong and attempt to help them understand how that particular kind of behavior is unacceptable. This clearly demonstrates that I, a 'being', am both just and merciful, in that I've been both just and merciful in my dealings with my children at different times. This is enough to serve as proof that the original statement - 'A being cannot be both just and merciful' - is simply false.
Let's deal with the ambiguity angle. One way to more clearly articulate here (not to be too hard on the original author for not being more verbose, I get that it it was twitter and he has a limited number of characters to work with) would be to say the following:
A being cannot be both just and merciful at the same time and in the same context. They are contradictions. Mercy is the suspension of justice.The only difference here is the "at the same time and in the same context" portion of the first sentence, a snippet taken directly from the Law of Non-Contradiction.
If this had been the way the conclusion had been stated, I couldn't help but agree with it. It seems obvious, that with it put more clearly in that manner, that it is true. Now that we've agreed that the more verbose and clear statment is true (no one has actually agreed with me, I'm just presuming that the original author would), my follow up to that would be: So what?
By 'So What?' I mean to say that Christianity does not claim that God is both merciful and just at the same time and in the same context. For God to be that way would mean that He is both merciful and just with the same individual... which would mean that same individual would spend eternity in both Heaven (being shown mercy) and Hell (being shown justice). Clearly this is not possible, no more possible than it is for me to for me to turn both left and right (at the same time and in the same way) or be both wet and dry (at the same time and in the same way).
No Christian doctrine or theological understanding that I am aware of makes such a claim. It would be completely illogical, the same as if I had claimed to punish my child for lieing to me but also claimed to have shown the child mercy for the same exact act. This is obviously not possible, I either punished them justly or I dealt with them mercifully.
The claim Christianity does make is that God deals with some people mercifully and others justly, thus He is a being that is merciful when he deals mercifully and He is a being that is just when he deals justly.
Thorn out, irritant gone.