Infant baptism is as easy to define as it is to say. It is exactly what it says it is.
The difficulty comes when explaining what it means.
The false doctrine of baptismal salvation became widespread between 100-200 a.d. By around 300 a.d. the unBiblical practice of infant baptism was beginning to spread, and it was a direct consequence of the erroneous teaching of baptismal salvation! The "logic" goes like this: If baptism washes away sin and regenerates, then should it not be applied as early as possible in a person's life? Baptize babies so that their sin is washed away and they can go to heaven. Bible historians, however, assert that there is NO evidence of the practice of infant baptism during the first two centuries of church history!
A. Baptismal Salvation (Baptismal regeneration)
Some churches (based on their own church liturgy) believe that a child is "saved" at baptism. Therefore, a child should be baptized as soon after birth as is possible. There are several serious problems with this idea.
(1) Nowhere in the Bible is infant baptism practiced or commanded.
(2) What becomes of a child who dies before he or she can be baptized?
(3) The Bible is clear that the way to be saved is not via baptism (baptism is certainly not unimportant, but it is not essential to salvation).
(4) IF (hypothetical only!) baptism could save someone, the mode (method) of baptism would seemingly be very important and "sprinkling" or "pouring" not only contradict the example found in Scriptures but go against the very meaning of the word "baptizo" which is to immerse or submerge.
For example, https://www.presbyterianmission.org/what-we-believe/baptism/
B. Baptism as a sign of "the covenant". (See Covenant Theology)
Some churches do not necessarily believe that a child is automatically "saved" upon being baptized, but that the child is placed into a covenant relationship with God. This becomes somewhat of a moot point since a person who is "covered" by the covenant is as good as being saved and if baptism is what brings one into the covenant, then it is baptism indeed which effectively saves one. The covenant relationship associated with infant baptism is usually based on the circumcision of the Jewish males as evidence of God's covenant with the nation of Israel through Abraham. This associative relationship between circumcision and baptism is suspect on several grounds. For instance, why is it that not only male children are baptized, since it was only the male children that were circumcised? In addition, there is simply no Biblical support for the idea that God ever intended for infant baptism in the church to replace Jewish circumcision.
C. Biblical authority
Biblical support and Biblical authority for the practice of infant baptism is absent. The history of the practice may be addressed in some of the links below, but Biblical examples and Biblical authority for the practice cannot be supported. Most authority for this practice comes from (a) historical records that do NOT go back to apostolic times, (b) "church" authority in churches where the church's authority is deemed to be equal to or greater than that of the Holy Scriptures (The Bible), or (c) assumptions/inferences made about several Bible verses. It is our belief at BibleTruthForToday that salvation by infant baptism is "another gospel" - NOT the true Biblical Gospel. The Bible provides these warnings against another gospel:
2 Cor. 11:3-4 "But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ. For if he who comes preaches another Jesus whom we have not preached, or if you receive a different spirit which you have not received, or a different gospel which you have not accepted—you may well put up with it!"
Galatians 1:6-9 "I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another; but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed."
Churches which practice infant baptism:
Christian Reformed Churches
Free Methodist Churches
Reformed Church in America
Roman Catholic Churches