How do one get to be in Christ? Is it same as to be righteous before God? I consider Romans 8:1 as axiom.
Therefore, there is now no condemnation for those who are in Christ Jesus,
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That little word in is packed with profundity, significance, and comfort for believers in Jesus Christ.
One way of approaching this little word is via one of the many names for the Church Universal: the Body of Christ. A body, of course, is composed of many parts (viz., appendages and external and internal organs), and so it is with Christ's body, the church, of which He is the Head. All of the parts of the physical body are in the body and are thus joined to the whole body. Each part has its function, but they all function together. The main point of the analogy of the body can be summed up, I feel, in the expression
To be in Christ, then, is to be a part of His body.
Theologically, the word in signifies where a believer is, positionally in the eyes of God; namely, in Christ. To be in Christ is to be united with Christ in His righteousness.
We who are sinners saved by grace are anything but righteous. Moreover, Isaiah tells us that
In other words, even the best we have to offer God cannot outweigh in the divine scale of justice our uncleanness, filthiness, and iniquities. Along comes Jesus, however, and His righteousness tips the scale in the opposite direction, so to speak.
Think of a zillion-pound weight being on one side of the teeter totter. That's our sin. Our righteous deeds piled up to the sky on the opposite side utterly fail to budge the zillion-pound weight one scintilla.
Jesus' cross death, however, means that God, when He imputes our sin to Jesus and then in turn imputes Jesus' righteousness to us when we believe, tipped the teeter totter as far as it can go in the opposite direction, as if the zillion-pound weight were but a feather! Aren't you glad you're in Christ? There is no safer place to be in all the universe, because when God sees us, He sees us in His Son. And as Paul reminds us,
What does it mean to be in Messiah?
Consider this interaction between Jesus and his disciples in John 14:
Jesus reiterates his teaching:
To summarize, Messiah teaches his disciples that by result that he is in the Father,
The Greek word meno means to stay, remain, or dwell, and is often translated to the English abide, a cognate from Anglo-Saxon with the very same comprehensive definition. We should consider abide in to mean “remain in” or “continuously be in.”
Jesus continues in John 15:
We see that the writers of the Epistles, namely Paul, also contemplated this notion of being in, as the state of being in Messiah is given significant consideration in the New Testament:
These numbered lists are of qualities and effects that result from being in Messiah, but do not determine what it is to be in Messiah. Paul gives a concise definition of what it is to be in Messiah:
To be in Messiah is to have Messiah abiding in you, to have him living through you. It is to be one with Messiah, like Messiah is one with the Father, and the Father is in Messiah, and the Father lives through Messiah. Again, Jesus says:
What Messiah says here of the Father is the same thing Paul says in the previous verse (Gal 2:20) of Messiah.
How does one get to be in Messiah?
Calling. Those who are in Messiah call upon Messiah and are called by God to be in Messiah.
Election. Those who are in Messiah were chosen by God to be in Messiah.
Faith. Those who are in Messiah have heard and believed the Gospel of Jesus, who is Messiah.
Obedience. Those who are in Messiah keep his words.
An important note to end on
In addition to the notion of being in presented here, there is an ancestral notion of being in. For example, Levi is described by the Hebrews author as in Abraham (literally, "in his body" or "in his loins") in Hebrews 7:9-10:
Levi was in Abraham, and also in Adam, as all are in Adam. This is significant to this question as the two aspects are contrasted by Paul in 1 Cor 15:22,
It is notable that the writer of the Epistle to the Hebrews applies this reasoning to a payment that was required under the law. Paul applies the same logic to the aspect of being in addressed in detail above, and describes it as the way for both justification of the sinner according to the payment required under the law of sinners, namely death, and also sanctification of the flesh.
It's just an alternative way to express that you have accepted Christ as Savior. It generally is used communally --"together in Christ" -- representing the way Christian belief unifies us.