What does it mean to confess Christ with our mouth?

Somebody asked this very pertinent question. I thought that Albert Barnes, in his commentary, did a very good job of explaining it. So here it is:

Because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. (Rom 10:9-10)

That if thou shalt confess - The word here rendered “confess” ὁμολογέω homologeō is often rendered “profess”; Mat 7:23, “Then will I profess to them, I never knew you;” Tit 1:16; Tit 3:14; Rom 1:22; 1 Tim 2:10; 1 Tim 6:12-13, 1 Tim 6:21; Heb 3:1, etc. It properly means to “speak what agrees with something which others speak or maintain.” Thus, confession or profession expresses our “agreement or concord with what God holds to be true, and what he declares to be true.” It denotes a public declaration or assent to that, here expressed by the words “with thy mouth.” A profession of religion then denotes a public declaration of our agreement with what God has declared, and extends to all his declarations about our lost estate, our sin, and need of a Saviour; to his doctrines about his own nature, holiness, and law; about the Saviour and the Holy Spirit; about the necessity of a change of heart and holiness of life; and about the grave and the judgment; about heaven and hell. As the doctrine respecting a Redeemer is the main and leading doctrine, it is put here by way of eminence, as in fact involving all others; and publicly to express our assent to this, is to declare our agreement with God on all kindred truths.

With thy mouth - To profess a thing with the mouth is to speak of it; to declare it; to do it openly and publicly.

The Lord Jesus - Shalt openly acknowledge attachment to Jesus Christ. The meaning of it may be expressed by regarding the phrase “the Lord” as the predicate; or the thing to be confessed is, that he is Lord; compare Acts 2:36 and Phil 2:11, “And that every tongue should confess that Jesus Christ is Lord.” Here it means to acknowledge him as Lord, that is, as having a right to rule over the soul.

Shalt believe in thy heart - Shalt sincerely and truly believe this, so that the external profession shall correspond with the real, internal feelings. Where this is not the case, it would be hypocrisy; where this is the case, there would be the highest sincerity, and this religion requires.

That God hath raised him - This fact, or article of Christian belief, is mentioned here because of its great importance, and its bearing on the Christian system. If this be true, then all is true. Then it is true that he came forth from God; that he died for sin; and that God approved and accepted his work. Then it is true that he ascended to heaven, and is exalted to dominion over the universe, and that he will return to judge the quick and the dead. For all this was professed and taught; and all this was regarded as depending on the truth of his having been raised from the dead; see Phil 2:8-11; Eph 1:21; Acts 2:24, Acts 2:32-33; Acts 17:31; 2 Cor 4:14; 1 Cor 15:13-20. To profess this doctrine was, therefore, virtually to profess all the truths of the Christian religion. No man could believe this who did not also believe all the truths dependent on it. Hence, the apostles regarded this doctrine as so important, and made it so prominent in their preaching.

Thou shalt be saved - From sin and hell. This is the doctrine of the gospel throughout; and all this shows that salvation by the gospel was easy.

For with the heart - Not with the understanding merely, but with such a faith as shall be sincere, and shall influence the life. There can be no other genuine faith than what influences the whole mind.

Believeth unto righteousness - Believes so that justification is obtained. (Stuart.) In God’s plan of justifying people, this is the way by which we may be declared just or righteous in his sight. The moment a sinner believes, therefore, he is justified; his sins are pardoned; and he is introduced into the favor of God. No man can be justified without this; for this is God’s plan, and he will not depart from it.

With the mouth confession is made ... - That is, confession or profession is so made as to obtain salvation. He who in all appropriate ways professes his attachment to Christ shall be saved. This profession is to be made in all the proper ways of religious duty; by an avowal of our sentiments; by declaring on all proper occasions our belief of the truth; and by an unwavering adherence to them in all persecutions, oppositions, and trials. He who declares his belief makes a profession. He who associates with Christian people does it. He who acts with them in the prayer meeting, in the sanctuary, and in deeds of benevolence, does it. He who is baptized, and commemorates the death of the Lord Jesus, does it. And he who leads an humble, prayerful, spiritual life, does it. He shows his regard to the precepts and example of Christ Jesus; his regard for them more than for the pride, and pomp, and allurements of the world. All these are included in a profession of religion. In whatever way we can manifest attachment to it, it must be done. The reason why this is made so important is, that there can be no true attachment to Christ which will not manifest itself in the life. A city that is set on a hill cannot be hid. It is impossible that there should be true belief in the heart of man, unless it should show itself in the life and conversation. This is the only test of its existence and its power; and hence it is made so important in the business of religion. And we may here learn,

(1) That a profession of religion is, by Paul, made as really indispensable to salvation as believing. According to him it is connected with salvation as really as faith is with justification; and this accords with all the declarations of the Lord Jesus; Mat 10:32; Mat 25:34-46; Luk 12:8.

(2) there can be no religion where there is not a willingness to confess the Lord Jesus. There is no true repentance where we are not willing to confess our faults. There is no true attachment to a father or mother or friend, unless we are willing on all proper occasions to avow it. And so there can be no true religion where there is too much pride, or vanity, or love of the world, or fear of shame to confess it.

(3) those who never profess any religion have none: and they are not safe. To deny God the Saviour before people is not safe. They who do not profess religion, profess the opposite. The real feelings of the heart will be expressed in the life. And they who profess by their lives that they have no regard for God and Christ, for heaven and glory, must expect to be met in the last day, as those who deny the Lord that bought them, and who bring upon themselves quick destruction; 2 Pet 1:2.