A Christian is a label that identifies one with the Christian Church which is supposed to built and founded upon the principles and teachings of Jesus Christ. Many claim to be Christian and are not but the "wheat and tare will grow together"(Matthew 13:30). This shows that the outer look can be so similar to the naked eye that only one with a keen and discerning sight can determine those who are saved and those who are not. And that person is God (Matthew 13:30) yet He can reveals to His saints through the Holy Spirit (1 Corinthians 2:10).
Thus being a Christian or being saved, born again, converted, etc. is
1. Being called by God unto a great salvation (John 6:44; Hebrews 2:3; Romans 8:29-30)
2. Accepting that call and putting full belief, faith and trust in Jesus (John 3:16; Hebrews 10:23; Romans 10:17; John 14:1)
3. Obeying the Word of the Lord and the unction of the Holy Ghost (i.e. baptism, etc.) (Matthew 7:21; Luke 6:46; Romans 2:13; 1 Thessalonians 5:19)
4. Living a sanctified and holy life unto the Lord through the work of the Holy Ghost (2 Corinthians 7:1; Hebrews 12:14; 1 Peter 1:15; 1 John 3:3; Revelation 22:11)
5. Walking in the Holy Spirit (fruit and gifts, ministry, etc.) (Romans 8:14; Galatians 5:16-23; Ephesians 5:18; Jude 1:20-25; Acts 2:4; Acts 19:1-6)