Eph 1:13 In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise
RO 8:9 ...if anyone does not have the Spirit of Christ, he does not belong to Him.
The word "Christian" just means one who belongs to Christ and follows Christ.
"Christ" refers to the Lord Jesus Christ, the Messiah, or the Anointed One. "Ian" is a suffix from Latin which means belonging to or adhering to. Put them together "Christ" + "Ian"= One who belongs to Christ and follows Christ, Christian in short.
If you don't have the Spirit of Christ, the Holy Spirit, the verse says you don't belong to Christ, you are not a Christian.
You may have undergone the rite of baptism, you maybe someone who has memorized the Bible, you maybe someone who goes to church regularly, you maybe someone who prays every day but if you don't have the Holy Spirit in you, then you don't belong to Christ. You are not a Christian.
How can one have the Spirit of Christ? Look at Eph. 1:13 above. It happens when one hears about the Good News of Christ's salvation and then puts his trust in it and then and only then, can one have the Holy Spirit in him.
And it is only the person who has the Holy Spirit who can truly follow Christ.
