Where in the Bible does it say we have a sin nature?
The Bible does not say that we have a sin nature in those explicit words. Neither do we find "trinity," or "rapture" in Scripture, but the concepts are still present under different wording. In answer to your question, I suggest thoughtful reading of Romans 7 is in order, especially verses 14 and 15-25.
For we know that the law is spiritual: but I am carnal, sold under
sin. 15 For that which I do I allow not: for what I would, that do I
not; but what I hate, that do I. 16 If then I do that which I would
not, I consent unto the law that it is good. 17 Now then it is no more
I that do it, but sin that dwelleth in me. 18 For I know that in me
(that is, in my flesh,) dwelleth no good thing: for to will is present
with me; but how to perform that which is good I find not. 19 For the
good that I would I do not: but the evil which I would not, that I do.
20 Now if I do that I would not, it is no more I that do it, but sin
that dwelleth in me. 21 I find then a law, that, when I would do good,
evil is present with me. 22 For I delight in the law of God after the
inward man: 23 But I see another law in my members, warring against
the law of my mind, and bringing me into captivity to the law of sin
which is in my members. 24 O wretched man that I am! who shall deliver
me from the body of this death? 25 I thank God through Jesus Christ
our Lord. So then with the mind I myself serve the law of God; but
with the flesh the law of sin.—Romans 7:14-25 AV
In verses 5-13, Paul uses the PAST tense, but beginning with the 14th verse and going through to the end of the chapter, Paul uses the present tense (I will bring this up again later).
In verses 15-25 we have personal testimony from the Apostle Paul himself with regard to the sin nature. Though Paul never uses the term, “sin nature,” it is evident that he is speaking of that very thing, regardless of the verbiage in which it is couched. Paul testifies of his own personal conflict between his new man and his old man. Apparently Paul thought this topic vital enough to use his own life as an example in order for us to understand the ongoing conflict regarding personal sanctification.
The Law comes from God who is Spirit (John 4:24) and that law expresses God’s will for man's life on this planet. Paul said the problem is that, "I am unspiritual (“fleshy, made of flesh”).
While giving his personal testimony in 7:14–25, Paul always used the present tense whereas he had used the past tense (vv. 7-13). It is obvious that Paul was describing his present, personal, inner turmoil as a Christian with indwelling sin (read as sin nature) and the continuing efforts of that nature to regain control in his daily life.
While it is accurately declared that the unregenerate are, “sold under sin” (AV), but sin still resides in believers. And it is to be remembered that even believers are still subject to physical death which is sin’s penalty. Thus, indwelling sin (read as Sin Nature) always seeks to exert itself in the life of a Christian.
At the very start of his testimony, Paul confessed...
For that which I do I allow not...(AV)
Here I prefer the rendering of the CJB or JB, both of which have it as...
I do not understand my own behavior...
Paul was doing things that he knew were wrong (sin) yet, to his horror, he found himself doing them anyway. What he wanted to do did not get done, but what he held in contempt in his thoughts and understanding, THOSE were the things he did.
Again, this same statement may easily be made by an unregenerate person at the zenith of his grandest moral and ethical experiences; but it can also, with equal ease and veracity, be said by someone born-again. Paul was, without question, describing his experience as a believer at that time.
Paul agreed that the Law is good, and because he agreed with the goodness of the law, Paul's conclusion was...
It is no longer I myself who do it but it is sin living in me
(literally, “but the dwelling-in-me sin”).
Paul also concluded that...
"nothing good lives in me."
Paul qualified that statement “in me” by stating "in my flesh." We may understand this as a reference to the "sin nature" when compared with verses 5 and 25.
So as not to leave anything to doubt or question, Paul further stated...
For I have the desire to do what is good, but I cannot carry it out
Again, I fund the CJB more pungent here than the AV...
For I know that there is nothing good housed inside me—that is, inside
my old nature. I can want what is good, but I can’t do it!
This, indeed, is the heartfelt cry of believers universally. While we so desperately desire to right by our God and Savior, what we actually do is to participate in actions that we both inwardly and outwardly detest.
Verses 19 and 20 are, again, colored vividly by the anguish Paul experiences even at the time of his writing this epistle to the Christians in Rome...
For I don’t do the good I want; instead, the evil that I don’t want is
what I do! 20 But if I am doing what “the real me” doesn’t want, it is
no longer “the real me” doing it but the sin housed inside me. CJB
What believer has not wanted to be more understanding and less judgmental about someone or some group, only to find that when next in their presence, you are even less understanding and more judgmental than before. Paul here equates the new man, the regenerate man as, "the real me." Ever believer has experienced this duality of existence. On the one hand we love the things that speak to righteousness and holiness; we hate that which is unloving and lacking in grace, but on the other hand we fail to do the good things we love and end up doing the sinful things we hate. Paul says this is owning to the Sin Nature which is alive and well in all believers, the Apostle Paul notwithstanding.
Paul explains why these things are so, not just in his life, but in the lives of all Christians.
I find this law at work.
He is not speaking of the Mosaic Law, he speaks of a principle drawn from his own experience, and one shared by all believers.
This law (or principle) is the reality of the ever-present inclination toward evil in an individual whenever he wants to do good. Paul did not back down from the fact that...
In my inner being I delight in God’s Law (compare this with verse 25).
The “inner man” is also used 2 Corinthians 4:16 and Ephesians 3:16. The psalmist declares his delight in God’s Law in Psalm 119 (e.g., vs. 16, 24, 47; and Psalms 1:2).
Since our regeneration (New Birth), we have a new nature; a new capacity for loving the things of God. But Paul said he saw yet another law (principle) at work within him. This is the principle of sin, the "old man," the "Sin Nature." Paul referred to it as “sin living in me” (Romans 7:17, 20), “evil” right there with me (v. 21), this is rightly understood as “the sinful nature” (vs. 5, 18, 25).
This Sin Nature is continually at work doing two things:
- Waging a non-stop war against the law of the believer’s mind (the new man), and
- Attempting to make him a prisoner of the law of sin (Sin Nature) in his body (flesh).