Baptism is the step every believer takes in fulfilling complete obedience to Jesus' word proving that He is Lord of their life. This is done by full emersion in water in accordance to a person's faith. Christians are baptized into Christ Jesus, into His death. 

Going into the water symbolizes going into the grave and dying with Jesus. Dying to the old nature once for all. Comin out of the water symbolizes rising in the newness of life leaving the old nature behind and in the grave having been nailed to the cross. 

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Romans 6:3-7  Or do you not know that all of us who have been baptized into Christ Jesus have been baptized into His death?  4  Therefore we have been buried with Him through baptism into death, so that as Christ was raised from the dead through the glory of the Father, so we too might walk in newness of life.  5  For if we have become united with Him in the likeness of His death, certainly we shall also be in the likeness of His resurrection,  6  knowing this, that our old self was crucified with Him, in order that our body of sin might be done away with, so that we would no longer be slaves to sin;  7  for he who has died is freed from sin.
 

This is a believer baptism. One must believe and then be baptized. Infant baptism is beautiful and powerful. However, it does not qualify as a believer baptism because an infant can not believe. 

Acts 8:37  [And Philip said, "If you believe with all your heart, you may." And he answered and said, "I believe that Jesus Christ is the Son of God."]
 

Acts 2:38  Peter said to them, "Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit.