Death by crucifixion
When the Romans crucified a man it was because they deemed him to be guilty as far as his crime was concerned, and to be unfit for their society as far as his person was concerned. In a similar way (hence the use of the metaphor of crucifixion), God has deemed us to be guilty as far as our sins are concerned, and unfit for the society of heaven as far as our persons are concerned. He goes further, however, and does what the Romans could not do. Their justice was satisfied when a criminal was flung, dead, into a malefactors grave. God, however, is the God of Resurrection. Yet He has no interest in raising sinners to heaven with sin upon them. The only solution in harmony with His justice is for another, even His own Son, to take legal responsibility for the sins of men. This Christ did by bearing the wrath due to them, then going into death to show that the process was over, and issuing forth in resurrection to show that what He did on the cross at Calvary has been accepted by His Father. Romans 4.25 assures us that He was raised from the dead because the work He did on the cross to enable God to justify us was acceptable.
But His death had another purpose. "For the wages of sin (that is, the sin-principle in man) is death" (Rom 6.23). By accepting the consequences of what Adam had done, and fully satisfying Gods every demand against man, Christ died, and in so doing died to that sin, having no more to do with it because He had satisfied God with regard to it (Rom 6.10).
Now it is this latter aspect of things which deals with what a person is in himself. The only way his constitution as a sinner can be dealt with is for him to be associated by God with what happened to Christ. So it is that the apostle declares that believers have been crucified with Christ. What they are in Adam has been cancelled by His cross. That this is the case is signified by burial with Christ in water baptism, for only dead men are buried. The story is told of two party-loving girls who got saved. Meanwhile, their friends had arranged a party. They took an invitation to the girls in person. When they reached the flat they shared they found a notice on the door which read, "We cannot come to the party because we are dead". They had grasped the truth of Romans 6 that as far as the old life is concerned, with its Adam-like ways, the believer is dead.
Life with Christ
There is more, though, for believers live. Not, however, with the old life, but with the new, for they are associated with a risen Christ and the Spirit of God indwells them, and that Spirit acts on the principle that they have life in Christ Jesus (Rom 8.2).
A problem confronts us, however, for we find that as believers we still have an ability to sin. How can this be, since we are dead with Christ and risen to a place where sin cannot come? The answer is found when we note the apostles references to the body and the members of that body in Romans 6.12,13,19. It is true that our old man was crucified with Christ so that the body as the seat of sin might be destroyed (i.e. made of no effect), but it is also true that we have deliberately not to yield our members "servants to uncleanness and to iniquity", and just as deliberately yield our members "servants to righteousness" (Rom 6.19). For as long as we are in the body we shall have the capacity to sin because of the flesh, that self within us which still wants to live as sinners live. The remedy is to reckon true in practice what is, in fact, true in principle. Gods reckoning of us is that we are dead and no longer in the flesh, and we are to act on that, reckoning ourselves to be "dead indeed to sin, but alive unto God through Jesus Christ our Lord" (Rom 6.11). The power to do this is the Spirit of God who energises and motivates us to live a spiritual life, which is a life governed by His dictates.
Summarising, we can say the following.
As a result of Calvary
The old man is crucified, as far as God is concerned (Rom 6.6).
The new man is created in us by God on the basis of the fact that Christ has been made sin for us (Eph 4.24; 2 Cor 5.17).
Being in the flesh is not our position as far as God is concerned (Rom 8.9). Those who are "in the Spirit" are in the good of Christs death and resurrection.
As a result of conversion
The old man was put off in principle by us at conversion, and this was confirmed at baptism (Col 3.9; Rom 6.4).
The new man was put on in principle when we were converted and baptised (Col 3.10).
We crucified the flesh with its passions and lusts (Gal 5.24). This is an absolute statement, not an exhortation, so it applies to all believers. When we first believed we renounced sin and in principle finished with it by repentance.
As a result of living consistently
The old mans characteristics are put off in practice (Col 3.8).
The new mans characteristics are put on in practice (Col 3.12).
We live after the Spirit and not after the flesh, concentrating on the things the Spirit of God finds congenial (Rom 8.12-13).
The dove that Noah let out of the ark found no rest for the sole of her foot as long as the waters of judgment were in evidence. As soon as the "new" earth emerged she found a place to rest. If we find rest and satisfaction in the things of this world then we are living after the flesh, even though in principle we are not in the flesh. If, however, we concentrate our minds and affections on that in which the Spirit delights, then we shall be living after the Spirit.
To be continued.