‘The Lord loveth mercy and judgment; the earth is full of the mercy of the Lord’ (Psalm 32:5).
If the judgment of God, who renders precisely according to our deserts what is due to us for our deeds, should be by itself, what hope would there be?
Who of all mankind would be saved? But, as it is, ‘He loveth mercy and judgment.’
It is as if He had made mercy a coadjutor to Himself, standing before the royal throne of His judgment, and thus He leads each one to judgment.
‘If thou, O Lord, wilt mark iniquities: Lord, who shall stand it?’ (Psalm 129:3).
Neither is mercy without judgment, nor judgment without mercy. He loves mercy, therefore, before judgment, and after mercy He comes to judgment.
However, these qualities are joined to each other, mercy and judgment, lest either mercy alone should produce presumption, or judgment alone cause despair.
The Judge wishes to have mercy on you and to share His own compassion, but on condition that He finds you humble after sin, contrite, lamenting much for your evil deeds, announcing publicly without shame sins committed secretly, begging the brethren to labor with you in reparation;
in short, if He sees that you are worthy of pity, He provides His mercy for you ungrudgingly.
But, if He sees your heart unrepentant, your mind proud, your disbelief of the future life, and your fearlessness of the judgment, then He desires the judgment for you —
— just as a reasonable and kind doctor tries at first with hot applications and soft poultices to reduce a swelling, but, when he sees that the mass is rigidly and obstinately resisting, casting away the olive oil and the gentle method of treatment, he prefers henceforth the use of the knife.
Therefore, He loves mercy in the case of those repenting, but He also loves judgment in the case of the unyielding.
[…] ‘The earth is full of the mercy of the Lord’. Here [on earth] mercy is separated from judgment. The earth is full of only the mercy of the Lord, since His judgment is stored up for the appointed time.
Here, then, mercy is apart from judgment; indeed, He did not come ‘in order that He might judge the world, but that He might save the world’ (cf. John 3:17).
But there, judgment is not apart from mercy because man could not be found clean from stain, not even if he had lived for only one day (cf. Job 14:4-5 LXX).
[…] While we are on earth, we need mercy…. For, when we were dead by reason of our offenses and sins God, having mercy, brought us to life together with Christ (cf. Eph. 2:5).
Basil the Great (330-379): Homily 15 (on Psalm 32), 3-4, from Saint Basil: Exegetic Homilies, translated by Agnes Clare Way, Catholic University of America Press (The Fathers of the Church, vol. 46), pp. 232-234.