What are the real differences between the Old Testament (OT) and the New Testament (NT)?
Some say that God is different, that in the OT He's the God of wrath and in the NT He's the God of grace. But both testaments show both His wrath and His grace.
In both testaments, sins are addressed by substitionary sacrifices, so that's not the difference, either.
The eternal Jesus does "show up" in the OT a few times, in what we call "theophanies," so it's not the existence or appearance of Jesus.
Clearly, the Incarnation of Jesus is only in the NT, plus His teaching ministry, death, burial, and resurrection. And the Church didn't exist until the NT. So, those are big differences.
There is one key distinction that can perhaps summarize the difference between the testaments - the phrase "in Christ." Being in Christ is something that was not possible in the OT, only in the NT. The incarnation, death, burial, and resurrection are what makes being in Christ possible and meaningful. The Church is defined by those who are in Christ. So, this phrase "in Christ" could be one way to distinguish the NT from the OT.
"In Christ" describes a state of being - either you are or you aren't in Christ. One becomes in Christ through faith in Him, and when you're in Christ, things that apply to Him are applied to you - His righteousness, His name, His reputation, His inheritance, His death for sins, and His resurrection. We are made in Christ through the indwelling Holy Spirit, and being in Christ is scripturally contrasted with being "in Adam."
The NT is not new, better, harder, more grace-infused rules compared to the OT. The NT isn't a better way for God to approve of you. The NT isn't about getting your eternal ticket punched. The NT is about being in Christ.
We can live our lives now in Christ, in faith. When we do our work, we can do it in Christ (or we can do it in Adam). As we live in our neighborhoods near our neighbors, we can be a neighbor in Christ. We can spend time with our families in Christ. We can pay the monthly bills and shoot hoops at the gym in Christ.
We can live life more in Christ first by praying about it, asking God to help us live life in His Son rather than in Adam. We can also get in the habit of asking ourselves, "How can I do this in Christ in particular?" or even, "Can I do this in Christ?" We can develop a continual awareness that Christ in us can make a practical difference in what we do and how we do it. Furthermore, we can take a listening posture and ask, "Is Christ in me urging me to do something?"
We live life in Christ best by intentionality. We don't just say we are "in Christ" - we think about it, pray about it, and take purposeful steps to keep "in Christ" as a constant ethic.
Stop, take a breath, and honestly say, "In Christ, I take the next step forward in obedience."
(Image: By Ken and Nyetta - https://www.flickr.com/photos/71279764@N00/5249145754/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=35682473)