Objection 1. It seems that the justice of God is not truth. For justice resides in the will; since, as Anselm says (Dial. Verit. 13), it is a rectitude of the will, whereas truth resides in the intellect, as the Philosopher says (Metaph. vi; Ethic. vi, 2,6). Therefore justice does not appertain to truth.
Objection 2. Further, according to the Philosopher (Ethic. iv, 7), truth is a virtue distinct from justice. Truth therefore does not appertain to the idea of justice.
On the contrary, it is said (Psalm 84:11): "Mercy and truth have met each other": where truth stands for justice.
I answer that, Truth consists in the equation of mind and thing, as said above (Question 16, Article 1). Now the mind, that is the cause of the thing, is related to it as its rule and measure; whereas the converse is the case with the mind that receives its knowledge from things. When therefore things are the measure and rule of the mind, truth consists in the equation of the mind to the thing, as happens in ourselves. For according as a thing is, or is not, our thoughts or our words about it are true or false. But when the mind is the rule or measure of things, truth consists in the equation of the thing to the mind; just as the work of an artist is said to be true, when it is in accordance with his art.
Now as works of art are related to art, so are works of justice related to the law with which they accord. Therefore God's justice, which establishes things in the order conformable to the rule of His wisdom, which is the law of His justice, is suitably called truth. Thus we also in human affairs speak of the truth of justice.
Reply to Objection 1. Justice, as to the law that governs, resides in the reason or intellect; but as to the command whereby our actions are governed according to the law, it resides in the will.
Reply to Objection 2. The truth of which the Philosopher is speaking in this passage, is that virtue whereby a man shows himself in word and deed such as he really is. Thus it consists in the conformity of the sign with the thing signified; and not in that of the effect with its cause and rule: as has been said regarding the truth of justice.
Source: Summa Theologica (New Advent)