When was the first gospel sermon preached?  I would say it was on the day of Pentecost.  That is when Peter preached the first sermon after the death, burial, and resurrection of Christ (Acts 2:14-39).  However, it was not the first time the gospel was declared.  That came when God gave Abraham what we often call the Abrahamic promise.

Galatians 3:7-9 (ESV)
⁷  Know then that it is those of faith who are the sons of Abraham. ⁸  And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.” ⁹  So then, those who are of faith are blessed along with Abraham, the man of faith.

In telling Abraham that all nations would be blessed in him, God was setting forth a synopsis of His plan of salvation saying to Abraham, “in you” which meant in his offspring.  Paul was quoting God’s promise to Abraham in Genesis 12:3, but after Abraham proved his obedient faith, by showing his willingness to offer Isaac up as a sacrifice, God renewed His promise, saying, “and in your offspring shall all the nations of the earth be blessed, because you have obeyed my voice” (Genesis 22:18).

The offspring that God was referring to was Christ.

Galatians 3:16 (ESV)
¹⁶  Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ.

The message from God to Abraham was that salvation would be in Christ, and Paul referred to that as the gospel being preached beforehand.  Nothing has changed today.  Anyone who is going to be saved must believe and accept Jesus as their Savior.  The promised blessedness is not found near or around the offspring, but in Him.  In Christ is where all spiritual blessings are found.

Ephesians 1:3 (ESV)
³  Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places,

These spiritual blessings begin with being adopted into God’s family where we are made new.

2 Corinthians 5:17 (ESV)
¹⁷  Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.

Why are we a new creation in Christ?  It is because, in the process of being translated there, our sins were cleansed by the blood of Christ.  Being justified, we find ourselves in a state free from condemnation  (Romans 8:1).   If you obey the gospel, the words of God to Abraham will be fulfilled in you, and you will become a part of Abraham’s offspring, fully connected to Christ.

Galatians 3:29 (ESV)
²⁹  And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.

As you wind down for the night, think about these things