Defining Baptism as an ordinance

Matthew 28:19  “Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit,

What is an ordinance?
Baptism will sometimes be referred to as an "ordinance".  Why? The word "ordinance" comes from the Latin term ordinis that refers to a statement of order, law or decree from an authority.  An ordinance entails three characteristics: First, the practice must have been taught by Christ in the Gospels, practiced or preached in the Book of Acts and practiced or preached in the New Testament Letters.  The second trait of an ordinance is that it pictures the Gospel.  In the New Testament there are two such practices that fit this definition: Baptism and the Lord's Supper.  Then finally, an ordinance, being a decree, is issued from Christ to His church. 
Baptism is an ordinance because it is practiced and preached throughout the New Testament
In the opening of all four Gospels, Jesus is Baptized (immersed) by John the Baptist in the Jordon River.  John then spoke of how Jesus would later on baptize believers with the Holy Spirit and with fire. (Matthew 3:11; Mark 1:4,8).  Baptism's primary meaning is that of immerse, with its secondary meaning being that of "brought into connection or indentification with someone".  Thus John was speaking of the Holy Spirit's ministry of uniting believers with Jesus Christ in the New Covenant reality of the church. (Acts 2:1-4; 1 Corinthians 12:13)

As we come to the Book of Acts, Baptism is practiced and taught throughout the early church.  In passages such as Acts 2:41; 16:33; 18:8 and 22:16, we see believers, new converts, being baptized and then received into membership in the local church. 

Then in terms of the New Testament Letters (Epistles), we see Paul refer to Baptism in his writings (Galatians 3:27; Romans 6:4-11; 1 Corinthians 12:13; Colossians 2:11).  Peter too mentions Baptism in 1 Peter 3:18-21.  These texts are the main places I take new converts to in explaining the practice and meaning of Baptism.

Baptism is an ordinance because it proclaims the Gospel in picture form
When Christ was setting the foundations for the New Testament Church, he designated the Gospel to be proclaimed verbally through preaching and teaching, and pictured by means of the ordinances of Baptism and Lord's Supper.  For example, Baptism pictures the "putting away of the life, and putting on the new life".  (Galatians 3:27; Colossians 2:11-12)  Jesus speaks of the New Birth experience in John 3:3-6.  Also too, the Apostle Paul shows how Baptism pictures our prior identification with Christ in His Death, Burial and Resurrection, something of which occupies over 30% of the content of the four Gospels. (Romans 6:4-11)  The Apostle Peter speaks of Baptism picturing the pledge of a good conscience and the removal of dirt from the conscience. (1 Peter 3:18-21)  Hebrews 9:14 tells us that the Blood of Christ at salvation cleanses the conscience. 

We must understand that Baptism is "picturing" that supernatural change that occurs at salvation, rather than it being part of the actual salvation experience itself.  Baptism does not bring about salvation, rather it is an essential picture of that inward reality.  Baptism is an ordinance, a sign, signifiying externally a prior invisible reality that occurs by grace through faith in Jesus Christ.

Baptism is an ordinance because Christ gave it to His church
In all of the passages cited above, not one of them occur apart from connection with or practice of Baptism in the local church.  Christ issued forth, "ordained" baptism to be administered through the agency of the local church.  The Apostles and later leaders in the  church administered the rite.  Even though Baptism is commonly administered by ordained Pastors, the focus is not so much on who is Baptizing.  On occassion I've heard of Deacons, or Elders, or even rarer fathers Baptizing their converted children.  The chief emphasis of our point here is: under what context is the Baptism being administered?  As long as Baptism is performed in conjunction with the local church, the Lord's purposes for this practice are being kept.