A Brief Proof of The Invalidity of All Non-Baptist Baptism
The discussion below presents in brief form the Biblical fact that all non-Baptist baptism is invalid. The presuppositions below are developed at greater length elsewhere.
1.) All “baptisms” that are thought to convey saving grace are invalid. Scripture teaches justification by repentant faith alone, apart from any sacraments (Romans 3:28). The idea that baptism conveys saving grace is an accursed false gospel (Galatians 1:8-9) and an abomination to God. Since proponents of this heresy are hell-bound and their religious organizations are not true churches but are under the wrath of God, their baptisms are of necessity invalid. Thus, the “baptisms” of Roman Catholicism, Eastern Orthodoxy, Lutheranism, the “Church of Christ”/Campbellism, Mormonism, and all other religious organizations that claim that baptism remits sin cannot truly baptize.
2.) All “baptisms” that are not the immersion of a believer in water are invalid. Scripture teaches that baptism is immersion as a picture of the believer’s union with Christ’s death, burial, and resurrection (Romans 6:3-4), and saving faith is a prerequisite to baptism (Acts 8:36-38). Therefore all infant “baptisms,” as well as all “baptisms” by pouring or sprinkling, are invalid. Thus, the “baptisms” of western and eastern Catholicism and of the great majority of Protestantism are again shown to be invalid.
3.) All “baptisms” not administered by a true church are invalid. Baptism is a church ordinance—Christ gave the church as an institution, not the Apostles, or Christian individuals as such, the authority to baptize (Matthew 28:19-20). Baptism adds one to the membership of the congregation that authorizes the ordinance (Acts 2:41, 47; 1 Corinthians 12:13). Christ started His church, His assembly of baptized believers, in the first century, and promised that true churches would exist until His return (Matthew 16:18; Ephesians 3:21), practicing the ordinances of baptism (Matthew 28:19-20) and communion (1 Corinthians 11:26). Roman Catholicism and Eastern Orthodoxy did not exist in the first century, and all Protestant denominations did not exist for the first three-fourths of church history. Only Baptist churches have existed from the first century until the present age, in fulfillment of Christ’s promises. Since Catholic and Protestant religious assemblies are not the churches founded by Jesus Christ, they did not receive authority to baptize from Him in the Great Commission of Matthew 28:19-20, and therefore all their baptisms are invalid. Even when religious organizations such as Bible churches, Pentecostal congregations, or non-denominational churches practice the immersion of a believer in water, no Biblical baptism has taken place, as the Bible church, Pentecostal, and non-denominational movements did not exist for the overwhelming majority of church history, and thus their assemblies certainly cannot be the churches founded by Jesus Christ. Since the church was given the authority to baptize, not Christian individuals, the invalidity of non-Baptist baptism remains even if both the individual performing the immersion and the one being dipped are Christians. It is thus clear that all non-Baptist baptisms are invalid, having no Divine authority, and that no one who has not been immersed upon the authority Christ gave to His first church in the first century, and to the Baptist churches that succeeded that first church, has been validly baptized.
Since all non-Baptist baptisms are invalid, all those who have been truly converted should leave all other religious organizations (2 Corinthians 6:14-7:1; 2 Thessalonians 3:6, 14) and be baptized into the membership of a Bible-believing and practicing Baptist church, and faithfully serve the Lord Jesus Christ in His true church, His sole institution for public worship in this age (Acts 2:41-47; Hebrews 10:25). Furthermore, Baptist churches should reject as invalid all non-Baptist baptisms and accept into membership only believers who have been immersed by their own church or by other true Baptist churches.
 See History of Baptists, John T. Christian (https://faithsaves.net). “Baptist” is defined in this sentence as all churches which have come into existence from the original congregation founded by Jesus Christ—thus, for example, a medieval Waldensian church would be considered “Baptist,” while a religious organization such as a modern non-denominational congregation or a Bible church that agreed to place the name “Baptist” on its church sign would not be considered truly Baptist until the members of such a congregation received immersion on the authority of an already constituted assembly in the line of Christ’s true churches.