1689 LBC 26.1-2
1. The catholic or universal church, which (with respect to the internal work of the Spirit and truth of grace) may be called invisible, consists of the whole number of the elect, that have been, are, or shall be gathered into one, under Christ, the head thereof; and is the spouse, the body, the fulness of him that filleth all in all.
2. All persons throughout the world, professing the faith of the gospel, and obedience unto God by Christ according unto it, not destroying their own profession by any errors everting the foundation, or unholiness of conversation, are and may be called visible saints; and of such ought all particular congregations to be constituted.
1689 LBC 29.1-2
1. Baptism is an ordinance of the New Testament, ordained by Jesus Christ, to be unto the party baptized, a sign of his fellowship with him, in his death and resurrection; of his being engrafted into him; of remission of sins; and of giving up into God, through Jesus Christ, to live and walk in newness of life.
2. Those who do actually profess repentance towards God, faith in, and obedience to, our Lord Jesus Christ, are the only proper subjects of this ordinance.
I've seen some disparate theological arguments from some of my beloved Baptist brethren. For that reason I want to offer some thoughts for consideration and discussion so that we may glorify God by rightly understanding what is required of us in scripture.
Does a person enter into the visible church through water baptism? This is a different question than whether baptism is a sign of visible church membership. The question, and it's answer, is important to Baptists. We know what our Presbyterian brethren believe in this regard. Much to our satisfaction they readily confess that baptism does not confer saving faith. They believe it is a sign of what already is; namely that an infant born into a believing family is a member of the covenant family. Baptism is, to them, a sign of the thing signified, whether or not the infant is/will/will not display evidence of saving faith. As for Baptists, specifically confessional Reformed Baptists, what do we believe?
Invisible precedes the visible
Reformed Baptists (abbrev. "RB") believe that scripture teaches God's covenant of redemption is made with His elect, chosen by God from eternity past (Eph. 1:4). Old Testament saints were saved by faith (Gen. 15:6; Rom. 4:3) in the same manner as New Testament saints (1 Cor. 10:1-4).
RB's believe that the covenant sign of circumcision was initiated with Abraham, and continued to the time of Christ, was a spiritual sign and also national identification. It was spiritual in nature because it pointed to Christ. It displayed national identification because it was administered only to males born into the nation of Israel. The male receiving the sign was not saved by circumcision itself; instead it was to serve as constant reminder of Israel's separateness unto God.
RB's believe in a discontinuity between the physical sign of the Abrahamic Covenant and the New Covenant. While God has always had one called out people of faith, He dealt with Israel in a specific manner so that He may display His glory to all the nations. During the period of the pre-Messianic diaspora, circumcision was still a sign of the spiritual promise and of national identification. When Christ, the Messiah, was born in time, He embodied both the spiritual promise of circumcision and it's national identification. Christ was Prophet, Priest, and King. RB's believe there was an organic change in how God's called out people would be assembled and governed (Eph. 1:9, 10; 1:22, 23). Within a relatively short period of time during the time of the Apostles, circumcision would cease to be practiced among Jewish believers and be replaced with baptism. But unlike the sign of the Abrahamic Covenant which symbolized a spiritual promise and national identity, baptism would symbolize a professed reality, namely that the promised shadow of circumcision was now realized in Christ on the basis of faith. Circumcision had a faith component, but it was not perfectly clear or effectual to the Jewish male receiving it. Baptism would become the sign of those who were effectually called and converted on the basis of faith. Faith would become the determinative agent in regards to baptism. Therefore, the invisible precedes the visible. Entrance into the invisible church is on the basis of faith alone. The 1689 LBC 26.2 rightly purports that saving faith allows the individual to be considered as a visible saint, not the basis of water baptism, but through spiritual baptism, or faith alone. Prior to regeneration (saving faith) the individual is as the Apostle Paul described himself:
Ephesians 2:1-3 And you were dead in your trespasses and sins, 2 in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. 3 Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest.
So, what is the condition of the saint prior to his regeneration? According to Paul our pre-regenerate condition was as "children of wrath." Except for elect infants dying in infancy (a topic for another thread), all of us are born into the dominion of the "prince of the power of the air" and are "children of wrath." It is for this reason that RB's do not apply the New Covenant sign of baptism to infants.
Is Baptism a sign of entrance into the visible church?
Yes. Entrance into the visible church, preceded by entrance into the invisible church, is on the basis of faith in Christ. Baptism is "an ordinance of the New Testament, ordained by Jesus Christ, to be unto the party baptized, a sign of his fellowship with him, in his death and resurrection; of his being engrafted into him; of remission of sins; and of giving up into God, through Jesus Christ, to live and walk in newness of life." It is a sign of what has already taken place, not of what might possibly take place. It is the sign of the New Covenant and is to be conferred to "those who do actually profess repentance towards God, faith in, and obedience to, our Lord Jesus Christ..."
Why do RB's consider baptism to be a sign of visible church membership when actual visible church membership is on the basis of faith? Baptism is an ordinance of the church. It is the first act of obedience that a new believer is to display (Acts 16:33). It is a sign of union with Christ. In that sense it is a sign of visible church membership, whereas actual visible church membership is on the basis of faith. The two are sometimes confused, so it is necessary to delineate the difference.