If all Christians have already yielded to Christ's lordship, why did Paul write Romans 12:1-2, commanding believers to make a once-for-all surrender?
Here's what Paul wrote:
I urge you therefore, brethren, by the mercies of God, to present your bodies a living and holy sacrifice, acceptable to God, which is your spiritual service of worship. And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what the will of God is, that which is good and acceptable and perfect.
Those words in no way imply that the Romans had not yet submitted to Christ. The phrase "present your bodies" is an aorist-tense verb in the Greek text. Some have tried to argue that it speaks of a first-time, once-for-all surrender. That is an overly simplistic understanding of the aorist tense. It is more accurate to interpret Paul's command as calling for decisive, deliberate, but continuous surrender.
Romans 6:17-18 proves that Paul knew those believers had "obeyed from the heart that form of doctrine which was delivered [to them]. Being then made free from sin, [they] became the servants of righteousness" (KJV). Obviously he was not later calling them to yield to Christ for the first time.
No one would argue that the command in verse 2 to "be transformed" negates the truth that those people were already transformed--that they were new creations in Christ (2 Corinthians 5:17).
The simple teaching of Scripture is that faith, surrender, and the transformed life are not one-time events. The Word exhorts believers to have faith, surrender, and obey continuously. That does not mean they've never done those things before. In this life we can never have too many encouraging reminders to apply God's Word more diligently. God Himself uses such commands to ensure our perseverance.