1. The Regulative Principle of Worship. The earliest Baptists separated from the Congregationalist churches that practiced infant baptism because they believed that the elements of public worship are limited to what Scripture commands. Because the Bible does not command infant baptism, the early Baptists believed that infant baptism is forbidden in public worship, and the baptism of believers alone is permitted. The regulative principle of worship limits the elements of public worship to the Word preached and read, the sacraments, prayer, the singing of Psalms, hymns, and spiritual songs, and whatever else the Scripture commands.
Many Baptists today have completely abandoned the regulative principle of worship in favor of entertainment oriented worship, consumerism, individual preferences, emotionalism, and pragmatism. Such Baptists have abandoned the very principle that led to their initial emergence. I doubt they have the right to identify as "Baptist," since they don't hold to the regulative principle, which is an historical precondition of Baptist existence.
Reformed Baptists believe that God made a covenant of works with Adam, which he broke and so brought condemnation on the whole human race. But God mercifully made a covenant of grace with His elect people in Christ, which is progressively revealed in the Old Testament and formally established in the new covenant at the death of Christ. The only way anyone was saved under the old covenant was by virtue of this covenant of grace in Christ.
Baptist covenant theologians believe they are more consistent than paedobaptist covenant theologians with respect to covenant theology's hermeneutic of New Testament priority. According to the New Testament, the Old Testament promise to "you and your seed" was ultimately made to Christ, the true seed. Abraham's physical children were included in the old covenant, not because they are all children of the promise, but because God was preserving the line of promise, until Christ, the true seed, came. Now that Christ has come, there is no longer any reason to preserve a physical line. Rather, all who believe in Jesus are sons of Abraham, true Israelites, members of the new covenant, and the church of the Lord Jesus. In both the Old and New Testaments, the "new covenant" is revealed to be a covenant composed of believers only, who are forgiven of their sins, and have God's law written on their hearts.
Baptists today who adhere to dispensationalism and new covenant theology have departed from their historical roots and from the hermeneutical vision of the organic unity of the Bible cast by their forefathers.
3. Calvinism. Because Reformed Baptists held to the covenant theology (federalism) of the 17th century, they were all Calvinists. When Adam broke the covenant of works, God cursed all human beings with totally depraved natures, making them unable and unwilling to come to Christ for salvation.
But God didn't leave the human race to die in sin; rather, in eternity past, God unconditionally chose a definite number of people for salvation and formed a covenant of redemption with Christ about their salvation. At the appointed time, Christ came into the world and obeyed the covenant of redemption, fulfilling the terms of the covenant of works that Adam broke. In the covenant of redemption, Jesus kept God's law perfectly, died on the cross, atoned for the sins of the elect alone, and rose from the dead, having effectually merited salvation for them.
God made the covenant of grace with His elect people in which He applies all the blessings of life merited by Christ in the covenant of redemption. The Holy Spirit mercifully unites God's chosen people to Christ in the covenant of grace, irresistibly drawing them to Himself in their effectual calling, giving them a living heart, a living faith and repentance, a living verdict of justification, and a living and abiding holiness, causing them to persevere to the end, all of which life-blessings are the merits of Jesus Christ, purchased in the covenant of redemption, applied in the covenant of grace.
Therefore, while justified believers are free from the law as a covenant of works to earn justification and eternal life, God gives them His law as a standard of conduct or rule of life in their sanctification. God's moral law, summarized in the 10 commandments, including the Sabbath commandment, is an instrument of sanctification in the life of the believer. Believers rest in Christ for their total salvation. Christ takes their burdens of guilt and shame, and His people take upon themselves the yoke of His law, and they learn obedience from a humble and gentle Teacher.
Historic Reformed Baptists were confessional. They were not bare "biblicists." They did not believe that individual church members or individual pastors have the right of private interpretation divorced from the historic teaching of the church. They believed that the Bible alone is sufficient for doctrine and practice, but they believed the Bible must be read in light of the church's interpretive tradition. The Reformed Baptists believed that their theology was anchored in the church's rich theological heritage and that it was a natural development of the doctrine of the church in light of the central insights of the Reformation (sola Scriptura: no baptizing infants; sola fide: only converts are God's people).
Many Christians today are of the opinion that they have the right to read the Bible independently and come to their own private conclusions about what it means without consulting the church's authorized teachers or the orthodox confessions of faith. But Scripture teaches that the church is the "pillar and support of the truth." The church as a whole is charged with interpreting the Bible and God has authorized teachers in the church throughout history. Therefore, while every individual Christian is responsible to understand Scripture for himself, he dare not do so without carefully studying and understanding what the great teachers of the past have taught about the Bible.
Reformed Baptists hold to the Second London Baptist Confession of 1689 because they believe it is a compendium of theology that best summarizes the teaching of Scripture in small compass.