Has He not also atoned for any failure to obey what He has commanded?
The Bible teaches that if you believe, repent of your sins,confess Christ, and are baptized for the remission of your sins, you will be saved. After that, you must continue to follow God, and if you stumble, and you will, you can gain forgiveness for those sins by repenting and asking God to forgive you. I gave you the verses, so you are more than welcome to look them up yourself.
You've no doubt heard this example before, but what if I as a believer were to stumble and commit a sin and then subsequently die before having repented and asked for forgiveness?
Clear and righteous questions like this one reaffirm the Reformed faith for me.
The answer is self-evident. Praise God, Christ has atoned for my sins of yesterday, today and tomorrow.
All that He has ordained comes to pass.
I am reminded of this from John Owen
The Father imposed His wrath due unto, and the Son underwent punishment for, either:
1. All the sins of all men.
2. All the sins of some men, or
3. Some of the sins of all men.
In which case it may be said:
1. That if the last be true, all men have some sins to answer for, and so, none are saved.
2. That if the second be true, then Christ, in their stead suffered for all the sins of all the elect in the whole world, and this is the truth.
3. But if the first be the case, why are not all men free from the punishment due unto their sins?
You answer, "Because of unbelief."
I ask, Is this unbelief a sin, or is it not? If it be, then Christ suffered the punishment due unto it, or He did not.
If He did, why must that hinder them more than their other sins for which He died?
If He did not, He did not die for all their sins!"
Justification - we have been saved from the penalty of sin [Past Tense]
Sanctification - we are being saved from the power of sin [Present Tense]
Glorification - we shall be saved from the presence of sin [Future Tense]