"...there is no other God besides Me, A just God and a Savior..." Isaiah 45:21
"...that He might be just and the justifier of the one who has faith in Jesus." Romans 3:26
"...the only begotten of the Father, full of grace and truth." John 1:14
These are just a couple verses which express the mystery of God's justice and God's mercy. What does it mean for God to be just? What does it mean for Him to be merciful? How can he be both perfectly right/just and perfectly gracious/merciful at the same time? The word "mystery" appears quite often in the New Testament, actually. "the mystery of the kingdom of God" Mark 4:11, "the wisdom of God in a mystery" 1 Corinthians 2:7, "the mystery of His will" Ephesians 1:9, "the mystery of Christ" Ephesians 3:4, "a great mystery" Ephesians 5:32, "the mystery of the gospel" Ephesians 6:19, "the mystery which has been hidden from ages" Colossians 1:26, "the mystery of God" Colossians 2:2, "the mystery of the faith" 1 Timothy 3:9. Truly, the faith is a mystery - it is hidden from those who do not understand it and still has yet to be revealed and understood in full by those who believe.
Luke 1:68, 74-75
"...He has visited and redeemed His people...
To grant us that we,
Being delivered from the hand of our enemies,
Might serve Him without fear,
In holiness and righteousness before Him all the days of our life."
"...He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him." 2 Corinthians 5:21