It is by the gospel that we are saved.

Jesus died for our sins, He was buried, and on the third day He rose again. This is the gospel by which we are saved. (1 Corinthians 15:1-4)

We repent of our sins, becoming dead to sin. We are buried in water baptism in the name of Jesus. We are resurrected to a new life in Christ when we are filled with His Spirit. This is the gospel by which we are saved. This is how we obey the gospel. (Romans 6:1-13)

When Peter preached on the Day of Pentecost, he preached the gospel, the death, burial, and resurrection of Jesus. (Acts 2:22-24)

He then preached how to obey the gospel.

"Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?

“Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.

“For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." - Acts 2:37-39 KJV

The people responded to Peter’s preaching of the gospel by obedience to the gospel.

"Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers." - Acts 2:41-42 KJV

Our salvation from the gospel is not complete until we believe AND obey the gospel of Jesus. (2 Thessalonians 1:8, 1 Peter 4:17)

"In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory." - Ephesians 1:13-14 KJV