The message to the unbeliever is, “in Jesus Christ is the forgiveness of sins”. Therefore the evangelist’s goal is to set Jesus Christ crucified [Galatians 3:1] before your hearers and call them to repent, from idolatry which leads to sin, and believe in the true and living God [1 Thessalonians 1:9].
You can take many scriptural paths to the cross, as evangelists like the Rev. George Whitefield did, but review sermons in Acts to see how the Holy Spirit emphasizes the forgiveness of sins in Christ.
The is the sermon from Peter to the Gentiles at Cornelius’s house in Acts 10. The significance of this sermon is that it is the one where God made known that the Gentiles would be saved too.
34 Opening his mouth, Peter said:
“I most certainly understand now that God is not one to show partiality, 35 but in every nation the man who fears Him and does what is right is welcome to Him.36 The word which He sent to the sons of Israel, preaching peace through Jesus Christ (He is Lord of all)— 37 you yourselves know the thing which took place throughout all Judea, starting from Galilee, after the baptism which John proclaimed. 38 You know of Jesus of Nazareth, how God anointed Him with the Holy Spirit and with power, and how He went about doing good and healing all who were oppressed by the devil, for God was with Him. 39 We are witnesses of all the things He did both in the land of the Jews and in Jerusalem. They also put Him to death by hanging Him on a cross. 40 God raised Him up on the third day and granted that He become visible, 41 not to all the people, but to witnesses who were chosen beforehand by God, that is, to us who ate and drank with Him after He arose from the dead. 42 And He ordered us to preach to the people, and solemnly to testify that this is the One who has been appointed by God as Judge of the living and the dead. 43 Of Him all the prophets bear witness that through His name everyone who believes in Him receives forgiveness of sins.” [Acts 10:34-43].
And this is Paul’s sermon in Acts 13:16-41 to the Jews and Gentiles in Pisidia Antioch on his first missionary journey:
16 Paul stood up, and motioning with his hand said,
“Men of Israel, and you who fear God, listen: 17 The God of this people Israel chose our fathers and made the people great during their stay in the land of Egypt, and with an uplifted arm He led them out from it. 18 For a period of about forty years He put up with them in the wilderness. 19 When He had destroyed seven nations in the land of Canaan, He distributed their land as an inheritance—all of which took about four hundred and fifty years. 20 After these things He gave them judges until Samuel the prophet. 21 Then they asked for a king, and God gave them Saul the son of Kish, a man of the tribe of Benjamin, for forty years. 22 After He had removed him, He raised up David to be their king, concerning whom He also testified and said, ‘I have found David the son of Jesse, a man after My heart, who will do all My will.’ 23 From the descendants of this man, according to promise, God has brought to Israel a Savior, Jesus, 24 after John had proclaimed before His coming a baptism of repentance to all the people of Israel.25 And while John was completing his course, he kept saying, ‘What do you suppose that I am? I am not He. But behold, one is coming after me the sandals of whose feet I am not worthy to untie.’
26 “Brethren, sons of Abraham’s family, and those among you who fear God, to us the message of this salvation has been sent. 27 For those who live in Jerusalem, and their rulers, recognizing neither Him nor the utterances of the prophets which are read every Sabbath, fulfilled these by condemning Him. 28 And though they found no ground for putting Him to death, they asked Pilate that He be executed.29 When they had carried out all that was written concerning Him, they took Him down from the cross and laid Him in a tomb. 30 But God raised Him from the dead; 31 and for many days He appeared to those who came up with Him from Galilee to Jerusalem, the very ones who are now His witnesses to the people.32 And we preach to you the good news of the promise made to the fathers, 33 that God has fulfilled this promise to our children in that He raised up Jesus, as it is also written in the second Psalm, ‘You are My Son; today i have begotten You.’ 34 As for the fact that He raised Him up from the dead, no longer to return to decay, He has spoken in this way: ‘I will give you the holy and sure blessings of David.’35 Therefore He also says in another Psalm, ‘You will not allow Your Holy One to undergo decay.’ 36 For David, after he had served the purpose of God in his own generation, fell asleep, and was laid among his fathers and underwent decay;37 but He whom God raised did not undergo decay. 38 Therefore let it be known to you, brethren, that through Him forgiveness of sins is proclaimed to you,39 and through Him everyone who believes is freed from all things, from which you could not be freed through the Law of Moses. 40 Therefore take heed, so that the thing spoken of in the Prophets may not come upon you:
41 ‘Behold, you scoffers, and marvel, and perish;
For I am accomplishing a work in your days,
A work which you will never believe, though someone should describe it to you.’”