Before we seek to answer your question, I want to quote three scriptures which your friend must be referring to. In Romans 3:10 & 23 we read, “As it is written: there is none righteous, no not one…For all have sinned and fall short of the glory of God.” In verse 10 God is teaching mankind that there is NOT ONE man, woman, boy or girl who is righteous in His sight. If they were RIGHTEOUS, they would never have sinned. But verse 23 teaches us that “all have SINNED and fall short of the glory of God,” which means that ALL have fallen short of the perfect standard of righteousness. What is God’s perfect standard of righteousness? Why it is God Himself. God is HOLY; He has never sinned, and He will never sin because He is incapable of sinning! Everyone has been born into this world a SINNER (see Psalm 51:5) and everyone has committed SIN. It matters not if you “sin less” than others do; the fact that you have committed “one sin” mean you have “fallen short of the glory of God.” This is why God declares emphatically, “There is none righteous, no not one.”

In Ephesians 2:8-9 we read, “For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast.” We had just considered how NO ONE is righteous” and that “ALL have sinned.” Because of this “ALL need to be saved.” Why? Because God is “holy and righteous and He must judge sin” (Romans 6:23 & Hebrews 9:27). Yet “God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have eternal life” (John 3:16). The sinner who BELIEVES IN JESUS CHRIST will not perish; he/she is SAVED and has ETERNAL LIFE. The believer sees that Jesus Christ was sent by God the Father to become the Savior and we learn in many scriptures that He paid the two-fold penalty of sin (DEATH and JUDGMENT) on the cross. Man could not save himself by anything he could do; he must be saved by the work that Christ did on the cross. The believing sinner is thus assured that he “has been saved by grace through faith.”

Now as to your question: Is it true that we are assured of our salvation even if we commit sin after we have accepted Jesus Christ as our Savior? We saw in John 3:16 that God promises the believer in Jesus Christ that you “will not perish” and that you “have eternal life.” God cannot lie (see Titus 1:2); He means what He says. That one verse alone gives the believer that absolute ASSURANCE OF SALVATION, though there are many more verses which give the believer this blessed assurance (see John 5:24; 6:37-40; 10:27-30; 1^st Peter 1:3-5; 1^st John 5:11-13).

It is true that the believer may still commit sin for we still have “SIN in us” (the SINFUL NATURE that we were born with; see Romans 7:13-20). Some deny this but we learn in 1^st John 1:8, “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” IF we sin, we are allowing the “sin in us” to rear its ugly head in an “outward act of sin.” Sin should not control us though for God has given the believer all the resources he needs to gain the victory over sin (please read and meditate on Romans 6:6-14; 8: 2-4; Galatians 5:16; Psalm 119:11).

Let me close by warning us of a terrible teaching. There are those who would say, “Since I am eternally secure in Christ, I can go and sin to my heart’s content, for I can’t lose my salvation.” They are guilty of using “grace as a license to sin.” The apostle had this to say to them: “What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it” (Romans 6:1-2). The believer would never be guilty of such language. The believer knows that they owe every blessing for time and eternity to the matchless GRACE OF GOD, and they would never use it as a license to sin.  (DO)  (584.5)