After His resurrection Jesus told His followers to:
 

Mark 16:15-16

“Go into all the world and preach the good news to all creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.”

Just as He identified with fallen humanity, Jesus expects those who respond to His love and forgiveness to identify openly and publicly with Him. It is not baptism which saves a person! He did not say, “whoever is not baptized will be condemned.” He said only that those who do not believe will be condemned. Faith in Jesus Christ alone is what saves an individual but the faith in a person’s heart needs to be expressed openly and without shame! That open identification begins at baptism. 

Jesus’ Command To The Church

Prior to Jesus’ going to heaven, after His resurrection, He gave the Great Commission to His disciples: 
 

Matt 28:19-20:

“Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you.”

The church of Jesus Christ was to go and bring the salvation message to people in every nation in the world. Those who responded to the message were to be baptized and then instructed in everything Jesus commanded. 

A secret faith which costs a person nothing was never part of the Lord’s plan for mankind. He said whoever denied Him before men He would also deny before His Father in heaven. 
 

The Apostle Peter’s Proclamation

As Peter preached to the masses of people assembled in Jerusalem on the day of Pentecost he said:
 

Acts 2:36-38

“‘Therefore let all Israel be assured of this: God has made this Jesus, whom you crucified, both Lord and Christ.’ When the people heard this, they were cut to the heart and said to Peter and the other apostles, ‘Brothers, what shall we do?’ Peter replied, ‘Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins...’ With many other words he warned them; and he pleaded with them, ‘Save yourselves from this corrupt generation.’ Those who accepted his message were baptized, and about three thousand were added to their number that day.”

In response to the crowd’s question about what they had to do to be saved Peter repeated the Lord’s command - repent and be baptized. They were to believe in their hearts and then openly declare their identification with Jesus Christ as their Lord by being baptized.

Baptism is the pledge of a good conscience toward God!

Speaking about the great Biblical flood out of which only eight people of the whole population of the earth were saved, the Apostle Peter said: 
 

1 Peter 3:21

“...this water symbolizes baptism that now saves you also-- not the removal of dirt from the body but the pledge of a good conscience toward God.”

He is saying the waters of that flood were symbolically like the waters of baptism. Though the rest of the world died in those waters, Noah and his family were saved. The waters of baptism don’t cleanse away our sin, but they are a pledge, or testimony of a good conscience toward God which our faith in Jesus Christ brings.