If that were true, that the 'sin nature' is no more after believing on Lord Jesus, then why... did Lord Jesus say to pray that part in red below?...
Luke 11:1-4
11 And it came to pass, that, as He was praying in a certain place, when He ceased, one of His disciples said unto Him, "Lord, teach us to pray, as John also taught his disciples."
2 And He said unto them, "When ye pray, say, Our Father Which art in heaven, Hallowed be Thy name. Thy kingdom come. Thy will be done, as in heaven, so in earth.
3 Give us day by day our daily bread.
4 And forgive us our sins; for we also forgive every one that is indebted to us. And lead us not into temptation; but deliver us from evil.
KJV
Obviously, according to Lord Jesus, we can still sin even after having believed on Jesus. Apostle John shows this in 1 John 1 also.
What was Apostle Paul really saying then, when he spoke that we should reckon ourselves dead to sin under Christ? He didn't say we could no longer sin. Reckoning ourselves dead to sin is one thing, still being able to commit sin is another.
Rom 6:7-13
7 For he that is dead is freed from sin.
The reason why the above is so, is because it is our flesh that causes most of our sins.
8 Now if we be dead with Christ, we believe that we shall also live with Him:
9 Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over Him.
10 For in that He died, He died unto sin once: but in that He liveth, He liveth unto God.
11 Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
The Greek for "reckon" means to 'impute, suppose, reason, think on'. It means to consider ourselves buried and raised in newness of life with Lord Jesus. That is about our spirit-soul that receives that newness while inside our flesh, it is not about our flesh. Our flesh is still able to sin, and this is what Paul then warns...
12 Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
13 Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God.
KJV
Paul would not have said that, "Let not sin therefore reign in your mortal body, that ye should obey the lusts thereof", if he meant we cannot sin anymore after having believed on Jesus Christ.
Those men who preach that we cannot sin anymore after believing on Lord Jesus Christ are preaching against God's written Word, because they leave out Apostle Paul's warning against continued sin for believers.
Brethren, it is so... easy to take a passage(s) in God's Word, and leave out others that goes with it, and create a whole new theme and doctrine. But it won't be a real Christian doctrine taught in God's Word, because God's Word doesn't leave the verses out those men do.
Why then would the devil love that false doctrine that those in Christ cannot ever sin again? It ought to be easy to figure out. It's because the devil wants... us to keep on sinning. And that is exactly what will happen if we just disregard the works of the flesh we may commit, which means to still sin. I see many brethren on the OSAS doctrine that are told they can no longer sin do exactly that, sin left and right with absolutely no regard for it whatsoever! That is not how Apostle Paul taught for us in Christ to live. He warned against that behavior (see 1 Timothy 1).