The book of Romans was written to saved people. They happened to live in Rome, but they were already saved people nonetheless. (See section 17).
ROMANS 1:1 Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God,
ROMANS 1:7 To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ.
Romans 10:9, therefore, does not tell anyone how to become saved (because they were already saved by the time that Paul wrote Romans 1:7). Romans 10:9 tells us how to remain saved. Here's what Paul actually wrote:
ROMANS 10:5 For Moses describeth the righteousness which is of the law, That the man which doeth those things shall live by them.
ROMANS 10:6 But the righteousness which is of faith speaketh on this wise, Say not in thine heart, Who shall ascend into heaven? (that is, to bring Christ down from above:)
ROMANS 10:7 Or, Who shall descend into the deep? (that is, to bring up Christ again from the dead.)
ROMANS 10:8 But what saith it? The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach;
ROMANS 10:9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
When Paul wrote this, he deliberately, almost exactly, quoted the Law of Moses. Moses wrote:
DEUTERONOMY 30:11 ¶ For this commandment which I command thee this day, it is not hidden from thee, neither is it far off.
DEUTERONOMY 30:12 It is not in heaven, that thou shouldest say, Who shall go up for us to heaven, and bring it unto us, that we may hear it, and do it?
DEUTERONOMY 30:13 Neither is it beyond the sea, that thou shouldest say, Who shall go over the sea for us, and bring it unto us, that we may hear it, and do it?
DEUTERONOMY 30:14 But the word is very nigh unto thee, in thy mouth, and in thy heart, that thou mayest do it.
Compare Romans 10:6-9 with Deuteronomy 30:11-14. The two say almost exactly the same thing, except that with the coming of Jesus Christ, we are justified by faith in His name ("...if you will confess with thy mouth...and believe in thine heart..."), instead of by the works of the Law ("...that thou mayest do it.") Moses wrote this to tell the Children of Israel how to stay "saved" (how to avoid the curses of the Law, for they had long before left Egypt behind, in Exodus 12). Paul (basically) quoted Moses and in context too! Paul wrote this to tell the Church how to stay saved, as well.
In the midst of the book of Romans, Paul discusses sin. This discussion about sin begins at Romans 5:11 (or thereabouts), and continues on through at least the beginning of Romans 8. Therefore, it is in the context of a discussion about sin in which Paul wrote the following:
ROMANS 6:1 What shall we say then? Shall we continue in sin, that grace may abound?
ROMANS 6:2 God forbid. How shall we, that are dead to sin, live any longer therein?
ROMANS 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
ROMANS 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
ROMANS 6:5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
ROMANS 6:6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
This scripture, like the rest of Romans, was written to saved people, so Paul mentioned baptism in the past tense (in Romans 6:3,5).
Let us take the time to parse this scripture, very slowly and deliberately. The apostle Paul wrote:
ROMANS 6:1 What shall we say then? Shall we continue in sin, that grace may abound?
ROMANS 6:2 God forbid. How shall we, that are dead to sin, live any longer therein?
Observation: These two verses (vv. 1-2) establish the immediate context of Romans 6:3-6. Paul is talking about sin.
ROMANS 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
ROMANS 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
Observation: Christ died so that our sins could be forgiven. We are made partakers of His death through water baptism. Just as Christ rose from the dead (and lived again), we likewise should live in "newness of life." Notice the "born-again" terminology in Paul's words when he said "newness of life." Paul was certainly aware of what Jesus had said:
JOHN 3:3 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
JOHN 3:4 Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?
JOHN 3:5 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
Paul understood good–and–well that the "water" mentioned in John 3:5 referred to water baptism, and was part of being "born again." (See section 3 for more details.) Therefore, he mentioned "baptism" (Romans 6:3) in direct association with "newness of life" (Romans 6:4).
ROMANS 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
ROMANS 6:5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
ROMANS 6:6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
Christ's death was by crucifixion, and was for the forgiveness of our sins. The likeness of the death of our old man is also "crucifixion for the forgiveness of sins" (Romans 6:5-6). We are buried in the likeness of Christ's death through water baptism (Romans 6:4-5). If we are buried in the likeness of His death, then (and only then) we will also be in the likeness of His resurrection.
Notice how the apostle Paul mingled sin, water baptism, and the death of Christ together. If we have any sins which remain, we cannot be saved. It is through baptism that we are planted in the likeness of His death, so that our "bodies of sin" would be destroyed. Having our "bodies of sin" destroyed (Romans 6:6) means having our sins forgiven. In other words, water baptism in the name of Jesus Christ is required for the forgiveness of sin. This is consistent with what Peter taught:
ACTS 2:38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
It is also consistent with what Ananias told Paul:
ACTS 22:16 And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.
As we have just seen in Romans 6, it is also consistent with what the same Paul, an apostle of Jesus Christ, taught us gentiles, as well.