My first problem was not in trying to find a verse to support the assertion that the Bible does indeed teach we have a sin nature, but in trying to decide which one of MANY to start with. I think I'll pick 1 John 1:8, since it's quite straightforward:
"If we say we have no sin, we are deceiving ourselves and the truth is not in us."
The Greek word for sin, hamartia, is in the singular and that use indicates it refers not to personal sins, but to a sin nature. (Another example of this usage is in John 16:8 -- the Holy Spirit convicts of sin -- singular; ie, the fact we are sinners by nature and in need of a savior. We are condemned not by the personal sins we commit, but because we are descended from Adam. Ro 6:18 says that "through one transgression there resulted condemnation to all men...")
But back to 1 John 1:8 -- Further evidence that this verse is saying Christians have a sin nature lies in the fact that John is the writer, definitely a Christian, and is including himself in this "we".
"If we say we have no sin (nature), we are deceiving ourselves, and the truth is not in us. If we (himself and other Christians) confess (name, acknowledge, cite) our sins (and here it's plural, referring to personal sins) He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness."
The verb of the greek word for confess, homologeo, is in the present tense, which is another thing that tells us this is not referring to something that happened at salvation. The present tense indicates ongoing, habitual action. Compare this to salvation verses, like Acts 16:30, Believe on the Lord Jesus Christ and you shall be saved... Believe is in the aorist tense, for a one time action with results that continue forever.
Of course 1 John is also the book that has the verse people quote as being the justification for the idea that we no longer have a sin nature, I John 3:9
"No one who is born of God practices sin, because His seed abides in him, and he cannot sin, because he is born of God."
"Born of God" and "He cannot sin" seem to be the salient phrases... But you can't throw out the verses in chapter 1 for the sake of the verse in chapter 3... and I John 1:8 - 10 says we do sin. As Christians. So what's going on here? If we are lying if we say we have no sin nature, and need to confess personal sins on a habitual basis, how is it we "can't sin" if we're born of God?
Well, because we have two natures. We have the old nature and we have the new nature. "Therefore if any man be in Christ he is a new creature." (2 Co 5:17) It is a divine nature (2 Pe 1:4) and thus cannot sin.
The new nature, which cannot sin is in the same body as the old nature, which can. The two are said in Galatians 5:17 to be at war with one another.
"For the flesh sets its desire against the Spirit and the Spirit against the flesh; for these are in opposition to one another, so that you may not do the things that you please."
Being in a status of opposition to God, the flesh is by definition sinful. (Note also that Jesus came, not "in the flesh," but "in the likeness of sinful flesh." Ro 8:3 He was the only man ever born without a sin nature.)
And earlier in Galatians 5 (vs 11) where Paul addresses his readers as "brethren," makes it clear he's giving this instruction to Christians.
Galatians 5:16 commands us to "walk by the Spirit and you will not carry out the desire of the flesh." In Ephesians we are commanded not to grieve or quench the Spirit, but to be filled with the Spirit. God wouldn't command us to a status that we already, permanently have, so being filled with the Spirit must not be a permanent condition. We choose whether it's going to be the Spirit ruling us or the flesh -- primarily through identifying the sin in our lives and executing the command in 1 Jn 1:9 to confess it to God.
In Romans 6 Paul laments the presence of his sin nature, a good twenty plus years after he was saved: the good he wants to do, he doesn't. The evil he wishes not to do, he does. He even says, flat out that "no longer am I the one doing it, but sin which indwells me." Who will deliver him from this wretched body of death?
That's pretty clear, too. He even repeats it in vs 20 "...I am no longer the one doing it, but sin which dwells in me." (And again, note the singular usage of sin, meaning sin nature)
Though some claim this passage is referring to Paul's pre salvation state, I would disagree. The pre-salvation Saul of Tarsus did not think of himself as a wretched man. On the contrary, in Philippians 3 he described himself as one who had every reason to put great confidence in his flesh: "circumcised the eighth day, of the nation of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; as to the Law, a Pharisee; as to righteousness which is in the law found blameless..." Oh. I forgot about the zeal. He thought he was hot stuff, in the Light, killing Christians for God. He didn't think he was wretched at all.
No I believe this lament in Romans is one he made as a Christian who had, in fact, come quite a ways along the road of spiritual growth. His very maturity no doubt made him much more aware of all the myriad ways he could and did fail, despite all that he knew.
My, I think I've run on too long. And I'm already thinking of more... positional and experiential truth, the nature of the original sin, the wicked going astray from the womb...
Maybe tomorrow. Or not. We'll see how the Lord leads.