Romans 10

As Paul teaches the saints in Rome, he seems to be consistently describing the need and relationship of both faith and righteousness. There is much written in the Old Testament of the importance of one's heart. In my mind, the heart is our most inward desires. Paul speaks of the revelations of the Prophets -

"...The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach." (New Testament | Romans 10:8)

The word of faith is the doctrine and gospel of Jesus Christ including the teachings and commandments He gave. So Paul tells the Roman saints -

"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." (New Testament | Romans 10:9)

In my mind, confess would be to testify of your knowledge and committment to the Lord Jesus Christ and then Paul adds another condition "and shalt believe in thine heart". Believe in ones heart would follow what Paul had previous written -

"For not the hearers of the law are just before God, but the doers of the law shall be justified." (New Testament | Romans 2:13)

"Believe in thine heart" then would suggest being "doers" of the commandments for Paul explains -

"For with the heart man believeth unto righteousness..." (New Testament | Romans 10:10)

In other words, one that "believes in ones heart" would be one that would do the works or strive to do the works of righteousness. Works of righteousness are keeping the commandments, ordinances, and covenants the Savior has taught.