There is what is called doctrines of grace amongst those who hold to Reformed Theology or Calvinism. Reformed Theology or Calvinism is nothing strange. Just as some describe themselves as Pentecostals or Charismatics, Reformed Theology or Calvinism is a branch of Christianity with specific beliefs from Scripture.
The doctrines of grace is acronised as TULIP: “Reformed Christians believe that all five of the doctrines of grace are derived directly from the Scripture and that the acronym TULIP accurately describes the Bible’s teaching on soteriology—the doctrine of salvation”. TULIP as an acronym represents: Total Depravity, Unconditional Election, Limited Atonement, Irresistible Grace And Perseverance of the Saints. A study of what these represents can begin from here.
Here is a brief explanation of the last letter of TULIP–P which is the focus of this article.
Perseverance of the Saints – The particular people God has elected and drawn to Himself through the Holy Spirit will persevere in faith. None of those whom God has elected will be lost; they are eternally secure in Him (John 10:27–29; Romans 8:29–30; Ephesians 1:3–14). Some Reformed theologians prefer to use the term “Preservation of the Saints” as they believe that this choice of words more accurately describes how God is directly responsible for the preservation of His elect. It is clear in Scripture that Christ continues to intercede for His people (Romans 8:34; Hebrews 7:25). This continues to provide believers with the assurance that those that belong to Christ are eternally His. 
Now, when I first became convinced of Reformed Theology, one of the doctrines of grace I grasped quickly was the last letter P–Perseverance/Preservation of The Saints. Some nickname it “Once Saved Always Saved” with a smirk of negativity though. Why I so quickly grasped it could probably be because I have had my own battles with doubt about my salvation and was desperately seeking for assurance to know I was indeed saved. What a joy to discover I couldn’t lose my salvation if indeed I was saved.
Assurance of salvation is the believer’s right and every believer must be “…the more diligent to confirm [their] calling and election” (1Peter 1:10). That said, it is crucial we don’t give ourselves and others false assurances of salvation if indeed we are not saved. The basis of assurance is for those “who are in Christ Jesus”. It is not for those outside Christ. For those in Christ, there is no condemnation. “There is therefore now no condemnation for those who are in Christ Jesus. (Romans 8:1). So first things first. A sinner must be saved before they can have any assurance of salvation. Though when saved, they might have seasons of doubt.
When we are saved, we must bear fruit. Yet, we don’t have to look at ourselves for assurance of salvation. Our human emotion is too frail and a bad barometer for measuring our assurance of salvation. What must be done? Look to God in His word. Seek and dig the word of God for your assurance because it is in God’s world we are told we are sinners and how we need to be saved and possess salvation.
Regardless of your feelings of doubt about your salvation, if you are truly saved, be assured “that he who began a good work in you will bring it to completion at the day of Jesus Christ” (Philippians 1:6). If you have it, that is, if you are saved, you can’t lose it. The believer is eternally secure.
1: Five Points Towards A Deeper Experience of God’s Grace by John Piper
2: Chosen By God by R.C. Sproul
3: Chosen For Eternity by John MacArthur
4: For Calvinism by Michael Horton
Other resources can be accessed from monergism.com a reliable place for Reformed resources