What is a simple biblical definition of Justification?
What constitutes mans justice?
The greatest commandments sum it up for us:
‘to love God with your whole heart, soul, mind and strength and your neighbor as yourself’. Matt 22:36-40.
Why?
Because: ‘
love fulfills the Law’. Rom 13:8-10
Why should love be the standard?
Because:
‘God is love’ John 4:8
And St Teresa of Avila, a Doctor of the Church, gives us further understanding:
“It is love alone that gives worth to all things.”
How is this accomplished?
By grace, won for us at Calvary, the gift of God’s Spirit who works in us and empowers us: **“With man this is impossible, but with God all things are possible.” **Matt 19:26
**Peter replied, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. 39 The promise is for you and your children and for all who are far off—for all whom the Lord our God will call.” **Acts 2:38-39
How else do we know? By the New Covenant promises:
33 “This is the covenant I will make with the people of Israel
after that time,” declares the LORD.
“I will put my law in their minds
and write it on their hearts.
I will be their God,
and they will be my people.
34 No longer will they teach their neighbor,
or say to one another, ‘Know the LORD,’
because they will all know me,
from the least of them to the greatest,”
declares the LORD.
“For I will forgive their wickedness
and will remember their sins no more.” Jer 31:33-34
**I will sprinkle clean water on you, and you will be clean; I will cleanse you from all your impurities and from all your idols. 26 I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh. 27 And I will put my Spirit in you and move you to follow my decrees and be careful to keep my laws. **Ez 36:25-27
**20 “The Redeemer will come to Zion,
to those in Jacob who repent of their sins,”
declares the LORD.
21 “As for me, this is my covenant with them,” says the LORD. “My Spirit, who is on you, will not depart from you, and my words that I have put in your mouth will always be on your lips, on the lips of your children and on the lips of their descendants—from this time on and forever,” says the LORD. ** Is 59:20-21
Also reference Matt 25:31-46. Sorry, it started out simple enough.

But, echoing the greatest commandments at the beginning of my post, St John of the Cross, another Doctor of the Church, summed it up most succinctly:
“At the evening of life, we shall be judged on our love.”