We can and do, sin while under God's grace.
First, it deeply saddens me that any believer would actually think this way. God's grace is not a license for immorality (Jude 1:4). God is good; And His people are good, too. For they are reflection of the character of God living in them.
Second, you know the good guys from the bad guys by what they do and not in them having a belief alone. For if an axe murderer had a family tied up and was about to kill them and said he believed in Jesus, I highly doubt that the family would believe he was a genuine Christian who was truly following the teachings of Jesus. Paul says if any man does not consent (i.e. does not agree) with the words of Jesus Christ and the doctrine according to godliness, he is proud and he knows nothing (See 1 Timothy 6:3-4). James 4:6 says God resists the proud and gives grace to the humble.
Three, in Matthew 7:23, we learn about certain believers who did wonderful works, but Jesus told them to depart from Him because they worked iniquity. Iniquity is a form of sinning. In fact, Jesus drives this point home even more and says that any man who does not do what He says is like a fool who built his house upon the sand, and when a storm came, great was the fall of that house (See Matthew 12:26-27).
You said:
Okay. This verse does not help you in the least bit. Maybe you meant 1 John 1:8? For most Christians I talk with use 1 John 1:8 as their excuse that they will always be in some kind of sin of some kind.
For does not 1 John 1:8 say that if we have no sin we deceive ourselves?
Answer: Well, what is helpful in understanding 1 John 1:8 is looking at it's immediate context. 1 John 1:10 says if we say we have not sinned. 1 John 1:10 moves verse 8 (which is present tense) into a past declaration (with verse 10). Verse 10 is saying there are people who said they have not sinned (past tense). This is clearly a gnostic belief. Most believers today hold to the idea that they have sinned at some point in their life (Regardless of whether they are an OSAS believer or a Conditional Salvationist). 1 John 1:8 is a present declaration of sin. It is saying if we say we have no sin when we do sin (present tense). This has to be the interpretative understanding of this verse because 1 John 2:4 says if we say we know Him and do not keep His commandments we are a liar and the truth is not in us. The OSAS's interpretation on 1 John 1:8 does not work because it conflicts with a normal reading on 1 John 2:3-4. You cannot always be in sin (breaking God's commands) as a part of 1 John 1:8 and yet also fulfill 1 John 2:3 that says we can have an assurance of knowing Him if we keep His commandments. Especially when 1 John 2:4 says we are a liar and the truth is not in us if we break his commandments. In other words, if the OSAS interpretation on 1 John 1:8 was true, then I would be damned if I do by obeying God's commands (1 John 1:8) and yet I would be damned if I don't by not obeying God's commands (1 John 2:4).
In fact, the New English Translation says this for 1 John 1:8,
"If we say we do not bear the guilt of sin, we are deceiving ourselves and the truth is not in us." (1 John 1:8 NET).
In other words, this verse is saying that if a person sins and says they do not bear the guilt of sin (in the sense that they will not have to face any wrath or Judgment from God over their sin) then they would be deceiving themselves and the truth would not be in them. This is exactly what the Eternal Security proposes. They are saying that they do not bear the guilt of any sin (destruction of their soul and body in hell fire) if they do sin because they believe their sins are paid for: Past, present, and future by Jesus. They are saying, they do not bear the guilt or the punishment of sin at the final Judgment because of their belief on Jesus. In short, 1 John 1:8 is a denial of the existence of sin on some level. “If we say we have no sin (in the sense that it does not exist) we deceive ourselves and the truth is not in us.” (1 John 1:8). Christian Scientists think sin is an illusion and does not exist at all. So this verse would apply to them. Eternal Security Proponents and those who deny that “Sin Can Separate a Believer from God” deny the existence of sin partially. They believe sin exists physically but they do not believe sin exists for them on a spiritual level because Jesus has forgiven them of all their sin by their belief on Jesus. In fact, to see just how silly your argument actually is for 1 John 1:8, you would have to believe that you are sinning right now at this very moment in order for such a verse to be true because 1 John 1:8 is speaking in the present tense.
In Romans 7:14-24, Paul is recounting his experience as a Pharisee before he became a Christian. Paul (Saul) is describing his experience of what it is like to struggle in keeping the Old Covenant Law that did not include Jesus Christ.
Romans 7:25 is a verse that transitions back to the present day reality as Paul being a Christian. He is thankful that he now has victory in Jesus Christ His Lord who can deliver him from his body of death (Which was a problem before). Otherwise why is Paul thanking Jesus?
While I have a total of 8 reasons that Paul is talking as a Pharisee in Romans 7:14-24, I am only going to give you two reasons (to keep things brief).
Reason #1:
Paul says in Romans 7:14 that he is carnal and is sold under sin; And yet in Romans 8:2, Pauls says he is free from sin. So unless Paul is contradicting himself, he is talking from two different perspectives.
Reason #2:
We are dead to the Law by the body of Jesus Christ (Romans 7:4). Would this be the Old Law or ALL law? 1 John 3:23 is a commandment that says we are to believe on the Lord Jesus Christ. This is a New Covenant Law. So obviously we are not dead to this Law or Command. The Scriptures also say, "but now commandeth all men everywhere to repent." (Acts of the Apostles 17:30). Are we dead to this Law? Surely not. Jesus said "repent or perish." (Luke 13:3). Peter told Simon to repent (by way of prayer to God) of his wickedness of trying to pay for the gifts of the Holy Spirit so that he may be forgiven (Acts of the Apostles 8:22). Sin is merely transgression of the Law (1 John 3:4). All this lets us know that men of God can break God's laws and they can be separated from GOD because of it. So surely some kind of Law of God is still in effect and has dire consequences for any person's soul who commits them. For Jesus said that if we do not forgive, we will not be forgiven by the Father (Matthew 6:15). If Jesus was talking to unbelievers, this would not make any sense. They would first need to accept Christ. So the only logical conclusion is that Jesus is talking to believers in Matthew 6:15. You do not forgive (i.e. you sin or break this law of God) and you will not be forgiven or saved. 1 John 3:15 says if you hate your brother you are like a murderer and no murderer has eternal life abiding in them. Again, you hate your brother (which can be a one time act) and you do not have eternal life. It's that simple. Also, Paul condemns circumcision several times. Galatians 5:2 is the biggest verse that condemns circumcision salvationism. Circumcision is an Old Covenant Law and it is not a New Covenant Law. Paul uses the word "law" when he speaks against circumcision. So we have to conclude that Paul is saying we are dead to the Old Covenant Law and not all Law. So again, this talk of the Old Law plays into verses 14-24.
You said:
We are all sinners and remain so in reality. It is only in God's eyes that we are no longer treated as such, because we are in Christ.
The Bible teaches that a believer cannot be in fellowship with God and also still be saved.
#1. 1 John 5:12 says He that has the Son has life and He that does not have the Son does not have life. Life is associated with eternal life or salvation.
#2. John 17:3 says eternal life is in knowing the one true God, Jesus Christ. Knowing implies a fellowship. So if you don't know Jesus, then you don't have life (Salvation).
#3. Romans 8:9 says if he a man does not have the Spirit of Christ, he does not belong to him.
#4. Psalms 73:27 says God will destroy all those who abandon Him (or go a whoring from Him).
#5. John 15:6 says if a man does not abide in Him, he is cast forth and burned.
#6. 1 John 1:7 says if we walk in the Light as He is in the Light, the blood of Jesus Christ cleanses us from all sin.
#7. Romans 11:21-22 says if you do not continue in his goodness you will be cut off. For if God spared not the natural branches (i.e. the Jews), take heed that he can do the same to you (i.e. Gentile believers). The analogy here is that you are branch and Christ is the tree. We need to continue in Christ's righteousness or goodness, not our own righteousness or goodness, or we will be cut off because of unbelief.