Justification: One and Done.
When I was about 10, I went on a hike with a boys brigade. We were all racing down this path at lunchtime when I decided to beat everyone to the bottom by deviating from the path. I ran straight down the side of the hill and received an immediate, involuntary lesson in physics.
My little legs couldn’t keep up with the momentum that was required to keep up and I ended up going down the hill Princess Bride “as you wiiiiiiiiish” style.
The protestant reformation took place 500 years ago – October 31, 1517 – as a result of false teaching that was sending the church running off to secure their salvation through good works – which not unlike my episode on that hike, starts you running down a path that proves utterly impossible to keep up.
The prevailing teaching in the medieval church was this: Yes, Christ’s work is necessary for salvation – but not sufficient for it.
The word “gospel” by it’s very definition it is not dependant on us because “news” takes place in the past. News is not something we can contribute to – news comes to us. The gospel is an announcement we receive, not an instruction we walk out. [a]
The grammatical construction of the NT consistently presents Christ is as the subject (the one doing the action) and the church is always the object (the one’s on the receiving end of His action). This means that all when the Scriptures call the church to obedience, that obedience cannot be for earning, completing, securing or ensuring. Our call to the obedience of Christ is explicitly for enjoying, glorifying, imitating, demonstrating and ministering.
After the reformation spread like wildfire, the church of Rome gathered a Council at Trent to set everybody “straight” in 1545. The papacy said in no uncertain terms that anyone who claimed that faith in Christ alone was enough for justification was cursed. 
Historically speaking, the religious response to grace has always been to put guardrails on it for fear that saying “Christ is enough” would either produce a lazy church or a wheels off sin-fest.
The apostles didn’t preach Christ with checks and balances on God’s grace. They never preached Christ “a lot” but Christ “alone”. They unapologetically proclaimed God’s rescuing grace while simultaneously teaching that His grace carried a reforming trajectory. [1b]
The cross of Christ takes away the guilt of sin hanging over you. That’s what the gospel is. The Spirit of Christ is increasingly loosening the grip of sin hanging onto you. That’s what the gospel does. 
We are saved by a perfect Redeemer – not our progressive reform.
When the scriptures teach about “justification” they are referring to a one-time act of God’s grace, for you. One and done. The apostles use this legal term to highlight that the just Judge over us is also One who justifies us. [2b]
When they speak about “sanctification”, they are referring to the ongoing work of God’s grace in you. Justification takes place outside you on Christ’s cross and sanctification takes place inside you by the Holy Spirit.
If we are not saved by a one-time act of grace outside us but by the ongoing work of grace inside us … then how much ongoing work is enough? This is why false teaching sucks the oxygen out of Christian faith and instead of engendering our hearts to live lives of obedience from sheer delight – it coerces us into a life of knuckle-dragging religious duty.
The gospel describes our relationship to God as a spiritual adoption.  Imagine a small child walking into the home of their adopted parents…
“Is this my house? … my food? … my room?”
“Yes” The Father says with a reassuring tone, smiling at the child.
The false gospel distorts this adoption, creating a very different image …
“Is this my house? … my food? … my room?”
“We’ll see” The father says, eyebrow raised. “I’m watching you.”
The love and good works that ensue from the adopted children in these two scenarios might look identical on the surface, but could not be motivated more differently beneath the surface. One seeks to obey and puts artwork on the fridge from love and acceptance, while the other does the same things for it.
Children of grace know their adoption is settled, legal and completed. They are free to spend their lives learning to grow into the family values of their adopted Father. Their hearts are at rest, so they live in compassion toward everyone else in the Fathers house.
Children of works are unsettled because they fear their adoption is pending. They walk out the family values in insecurity in the hopes that their good works will finalize their adoption. Their hearts are at unrest, so they live in constant comparison with everyone else in the Father’s house.
Contrary to popular belief, God’s primary goal as revealed in the scriptures is not getting better behaviour out of us – but justifying us in order to get His eternal life to us. I don’t say that to downplay or minimize Christian obedience – but for us to reimagine why we do it.
As those united to Christ, we have a completely different relationship with God’s law. It’s no longer a condemning search light that exposes us, it’s an illuminating spotlight that guides us.
That’s why those who are justified by the grace of Christ don’t live with indifference to the obedience of Christ.
Sunday would not be a day of rest if Christ’s obedience made our salvation possible but in the end it was the degree of our obedience that made salvation actual.
Our trust is not in Christ a lot – but in Christ alone. We could never do enough or be good enough – but Christ is enough and we’re in Him.
[a] Ephesians 2:4-9
 CANON XII.-If any one saith, that justifying faith is nothing else but confidence in the divine mercy which remits sins for Christ’s sake; or, that this confidence alone is that whereby we are justified; let him be anathema.
[1b] Titus 2:11-14
[2b] Romans 3:23-26