The bible teaches that after conversion, after being born again, a person now has 2 natures. This dual nature underlies our struggle as Christians to be purified and to “walk as He walked”.
After being born again a person is given a new nature. This new nature is the nature of God, the “seed of God”. We are “partakers of the divine nature” and “Jesus lives within us”. This new nature is contrasted with our “old nature”, and allows us to change and become “a new person”.
The old nature and the new nature coexist in the believer. They are not coequal however. The believer has power over the old nature and no longer has to obey it. That is, we “are dead to sin” and “sin shall no longer have dominion over us”. In fact, some people object to using the term “old nature” to refer to the forces tending towards evil that are retained after conversion. They (rightly) point out that after conversion their “essential nature” is their new nature. As long as you realize that you still have to resist sin after conversion then the terminology isn’t that important.
The old nature has to be resisted and nullified in order to manifest the new life within us. Thus we are told to “put off the old man” and “put on the new man”. We therefore read that we are to “mortify the members of our flesh” and “reckon yourselves dead to sin”.
A person must not only resist the old man, but they must “put on the new man”. Thus we find that if we “walk in the spirit we will not fulfill the works of the flesh”.
This crucifixion of the old man and the yielding to the new man are both active and passive. That is, some of this process depends on our choices and our use of our will. Much of this process though is passive. That is, not dependent on our choices or use of our will. We receive help from God that goes far beyond our own abilities and strength. To overlook the power of God and the indwelling Spirit will result in a person falling into legalism.
See also Double Minded.