What does it actually mean to describe God as a God of justice? 

We wouldn’t make much headway in such a solemn treatise if we fail to beam a light of reveal on the meaning the word.

The Oxford dictionary defines justice as the quality of being fair and reasonable; just behaviour or treatment or the administration of the law or authority in maintaining this.

So to say God is just is tantamount to saying He is fair and reasonable, behaves and treats everyone in a just way or that He administers His law and authority in this way.

Being just is tantamount to giving everyone the same treatment without fear or favour; making sure those that deserve punishment are punished and those that deserve blessing are blessed.

It also means administering them based on set principles, standards or law. So justice demands that anyone found wanting or fulfilling of the law, principle or standard should receive due reward for their behaviours.

Mercy could be defined according to the Oxford English dictionary as, compassion or forgiveness shown towards someone whom it is within one’s power to punish or harm.

The Merriam-Webster dictionary defines mercy as compassion or forbearance shown especially to an offender or to one subject to one’s power; a blessing that is an act of divine favor or compassion; a fortunate circumstance or a compassionate treatment of those in distress.

We see all in scripture how God is both just and merciful. We see Him administering justice, mercy and tampering justice with mercy alike. For instance justice demands that we should all die because we have all sinned and the wages of sin is death (Romans 3:23; 6:23). But God’s mercy made us right with God, redeemed and sanctified us by the blood of His own Son, Jesus Christ.

What judge is there that would give his own son to die in place of a criminal? 

Why not discharge and acquit the offender instead? But that would annul the fulfilment of the dictates of the law. Wouldn’t it be tantamount to injustice? If those who committed the same offence initially were not spared, wouldn’t it be injustice to just discharge and acquit this present offender?

Then mercy to the rescue.

God therefore not wanting any to perish (2 Peter 3:9;  Ezekiel 18:23; 33:11), gave His only begotten son as a repariation for our sins.

In iniquity we were conceived in our mothers’ wombs (Psalm 51:5). This is the original sin due to the sins of our ancestors (Adam and Eve). So if anyone says he has not sinned, he is fooling himself and he makes God a liar (1 John 1:8; 10). Apart from the iniquities of our we inherited