How then shall they call on him - As the apostle had laid so much stress on believing in order to salvation, and as this doctrine, without farther explanation, might be misunderstood, it was necessary to show how this faith was produced; and therefore he lays the whole doctrine down in a beautifully graduated order.

1. There can be no salvation without the Gospel: a dispensation of mercy and grace from God alone, here called, Romans 10:15, the Gospel of peace; glad tidings of good things.

2. This must be preached, proclaimed in the world for the obedience of faith.

3. None can effectually preach this unless he have a Divine mission; for how shall they preach except they be Sent, Romans 10:15. The matter must come from God; and the person mho proclaims it must have both authority and unction from on high.

4. This Divinely-commissioned person must be heard: it is the duty of all, to whom this message of salvation is sent, to hear it with the deepest reverence and attention.

5. What is heard must be credited; for they who do not believe the Gospel as the record which God has given of his Son cannot be saved, Romans 10:14.

6. Those who believe must invoke God by Christ, which they cannot do unless they believe in him; and in this way alone they are to expect salvation. Professing to believe in Christ, without earnest, importunate prayer for salvation, can save no man. All these things the apostle lays down as essentially necessary; and they all follow from his grand proposition, Whosoever shall call upon the name of the Lord shall be saved. But, says the apostle, How shall they Call upon him in whom they have not believed? And how shall they Believe in him of whom they have not heard? And how shall they Hear without a preacher? And how shall they Preach except they be sent? And with what message which can bring salvation can they be sent, but with the Gospel of Peace, the Glad Tidings Of Good Things. When, therefore, there is:

1st, a proper Message;

2ndly, a proper Messenger;

3rdly, the message Preached, proclaimed, or properly delivered by him;

4thly, the proclamation properly Heard and attentively considered by the people;

5thly, the message which they have heard, conscientiously Believed;

6thly, the name of the Lord Jesus, by whom alone this salvation is provided, most fervently Invoked; then,

7thly, salvation, or redemption from sin and misery, and the enjoyment of peace and happiness, will be the result of such calling, believing, hearing, preaching, sending, and message sent: - and thus the doctrine of salvation by grace through faith is guarded from abuse.

shall they.

1 Kings 8:41-43 Moreover concerning a stranger, that is not of your people Israel, but comes out of a far country for your name's sake...

Jonah 1:5,9-11,16 Then the mariners were afraid, and cried every man to his god, and cast forth the wares that were in the ship into the sea...

Jonah 3:5-9 So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth...

Hebrews 11:6 But without faith it is impossible to please him: for he that comes to God must believe that he is...

James 5:15 And the prayer of faith shall save the sick, and the Lord shall raise him up; and if he have committed sins...

and how shall.

Romans 1:5 By whom we have received grace and apostleship, for obedience to the faith among all nations, for his name:

Romans 16:25,26 Now to him that is of power to establish you according to my gospel, and the preaching of Jesus Christ...

Mark 16:15,16 And he said to them, Go you into all the world, and preach the gospel to every creature...

Luke 24:46,47 And said to them, Thus it is written, and thus it behooved Christ to suffer, and to rise from the dead the third day...

John 20:31 But these are written, that you might believe that Jesus is the Christ, the Son of God...

Acts 19:2 He said to them, Have you received the Holy Ghost since you believed? And they said to him...

Acts 26:17,18 Delivering you from the people, and from the Gentiles, to whom now I send you...

2 Timothy 4:17 Notwithstanding the Lord stood with me, and strengthened me; that by me the preaching might be fully known...

Titus 1:3 But has in due times manifested his word through preaching, which is committed to me according to the commandment of God our Savior;

Christ the End of the Law
The reason why many do not come to Christ is not because they are not earnest, after a fashion, and thoughtful and desirous to be saved, but because they cannot brook God's way of salvation. "They have a zeal for God, but not according to knowledge," We do get them by our exhortation so far on the way that they become desirous to obtain eternal life, but "they have not submitted themselves to the righteousness of God." Mark, "submitted themselves," for it needs submission. Proud man wants to save …
Charles Haddon Spurgeon—Spurgeon's Sermons Volume 22: 1876

Which Sentence Dishonoreth the Holy Martyrs, Nay Rather Taketh Away Holy Martyrdoms Altogether. ...
3. Which sentence dishonoreth the holy Martyrs, nay rather taketh away holy martyrdoms altogether. For they would do more justly and wisely, according to these men, not to confess to their persecutors that they were Christians, and by confessing make them murderers: but rather by telling a lie, and denying what they were, should both themselves keep safe the convenience of the flesh and purpose of the heart, and not allow those to accomplish the wickedness which they had conceived in their mind. …
St. Augustine—Against Lying

Or Haply is it So, that He who Plots in this Way to Find...
13. Or haply is it so, that he who plots in this way to find out Priscillianists, denies not Christ, forasmuch as with his mouth he utters what with his heart he believes not? As if truly (which I also said a little above) when it was said, "With the heart man believeth unto righteousness," it was added to no purpose, "with the mouth confession is made unto salvation?" [2398] Is it not so that almost all who have denied Christ before the persecutors, held in their heart what they believed of Him? …
St. Augustine—Against Lying