Reformed theology is generally considered synonymous with Calvinism and most often, in the U.S. and the UK, is specifically associated with the theology of the historic church confessions such as the Westminster Confession of Faith or the Three Forms of Unity.
Reformed theology holds to the authority of Scripture, the sovereignty of God, salvation by grace through Christ, and the necessity of evangelism. It is sometimes called Covenant theology because of its emphases on the covenant God made with Adam and the new covenant which came through Jesus Christ (Luke 22:20).
What does it mean to be Reformed?
A summary of Reformed theology, or what it means to be Reformed, may be seen in the following: 
- It means to affirm the great “Solas” of the Reformation. (See the Five Solas)
- It means to affirm and promote a profoundly high view of the sovereignty of God.
- It means to affirm the doctrines of grace. . . to see God as the author of salvation from beginning to end. The doctrines of grace are commonly referred to as TULIP. (See Calvinism) 
- Total Depravity – Sin has affected all parts of man. The heart, emotions, will, mind, and body are all affected by sin. We are completely sinful. We are not as sinful as we could be, but we are completely affected by sin.The doctrine of Total Depravity is derived from scriptures that reveal human character: Man’s heart is evil (Mark 7:21-23) and sick Jer. 17:9). Man is a slave of sin (Rom. 6:20). He does not seek for God (Rom. 3:10-12). He cannot understand spiritual things (1 Cor. 2:14). He is at enmity with God (Eph. 2:15). And, is by nature a child of wrath (Eph. 2:3). The Calvinist asks the question, “In light of the scriptures that declare man’s true nature as being utterly lost and incapable, how is it possible for anyone to choose or desire God?” The answer is, “He cannot. Therefore God must predestine.”Calvinism also maintains that because of our fallen nature we are born again not by our own will but God’s will (John 1:12-13); God grants that we believe (Phil. 1:29); faith is the work of God (John 6:28-29); God appoints people to believe (Acts 13:48); and God predestines (Eph. 1:1-11; Rom. 8:29; 9:9-23).
- Unconditional Election – God does not base His election on anything He sees in the individual. He chooses the elect according to the kind intention of His will (Eph. 1:4-8; Rom. 9:11) without any consideration of merit within the individual. Nor does God look into the future to see who would pick Him. Also, as some are elected into salvation, others are not (Rom. 9:15, 21).
- Limited Atonement – Jesus died only for the elect. Though Jesus’ sacrifice was sufficient for all, it was not efficacious for all. Jesus only bore the sins of the elect. Support for this position is drawn from such scriptures as Matt. 26:28 where Jesus died for ‘many’; John 10:11, 15which say that Jesus died for the sheep (not the goats, per Matt. 25:32-33); John 17:9 where Jesus in prayer interceded for the ones given Him, not those of the entire world; Acts 20:28 and Eph. 5:25-27 which state that the Church was purchased by Christ, not all people; and Isaiah 53:12 which is a prophecy of Jesus’ crucifixion where he would bore the sins of many (not all).
- Irresistible Grace – When God calls his elect into salvation, they cannot resist. God offers to all people the gospel message. This is called the external call. But to the elect, God extends an internal call and it cannot be resisted. This call is by the Holy Spirit who works in the hearts and minds of the elect to bring them to repentance and regeneration whereby they willingly and freely come to God. Some of the verses used in support of this teaching are Romans 9:16 where it says that “it is not of him who wills nor of him who runs, but of God who has mercy“; Philippians 2:12-13 where God is said to be the one working salvation in the individual; John 6:28-29 where faith is declared to be the work of God; Acts 13:48 where God appoints people to believe; and John 1:12-13 where being born again is not by man’s will, but by God’s.
“All that the Father gives Me shall come to Me, and the one who comes to Me I will certainly not cast out,” (John 6:37).
- Perseverance of the Saints – You cannot lose your salvation. Because the Father has elected, the Son has redeemed, and the Holy Spirit has applied salvation, those thus saved are eternally secure. They are eternally secure in Christ. Some of the verses for this position are John 10:27-28 where Jesus said His sheep will never perish; John 6:47 where salvation is described as everlasting life; Romans 8:1 where it is said we have passed out of judgment; 1 Corinthians 10:13 where God promises to never let us be tempted beyond what we can handle; and Phil. 1:6 where God is the one being faithful to perfect us until the day of Jesus’ return.
- It means to be creedal. . . to affirm the great creeds of the historic, orthodox church. (See e.g. the Nicene Creed)
- It means to be confessional. . . to affirm one or more of the great confessions of the historic orthodox church. (see e.g. the Westminster Confession)
- It means to be covenantal. . . to affirm the great covenants of Scripture and see those covenants as the means by which God interacts with and accomplishes His purposes in His creation, with mankind. (see Covenant Theology)
- It means to take seriously the Great Commission of Matthew 28:19-20. . . to affirm the primacy of mission and understand that mission.
- It means to have a distinctly Christian worldview that permeates all of life. (see Coram Deo)
- It means to evangelize. Reformed theology teaches that Christians are in the world to make a difference, spiritually through evangelism (the verbal proclamation of the Gospel) and socially through holy living and humanitarianism, such as taking care of the homeless, the widows, and the orphans.
- What does it mean to be Reformed? at federaltheology.org
- The Five Points of Calvinism. at https://www.calvinistcorner.com/tulip.htm
Reformed Theology Resources
- R. C. Sproul, What is Reformed Theology?
- Tim Challies, What it means to be Reformed.
- Orthodox Presbyterian Church, What is the Reformed Faith?