for me it is the full belief in the one and only Savior Jesus Christ. believing He was born of a virgin, lived a sinless life bore our sins on the cross and rose again the 3rd day. true chrisiantity is believing all those things

You could start with,

Christianity isn’t a religion, it’s a relationship. Christianity is a belief in God’s once, and for all time perfect atoning sacrifice. The sacrifice of His one and only begotten Son Jesus Christ the righteous . . . Then inform them, religion is mans attempt at reaching out to God . . . Christianity Is God reaching down to sinful man and offering him eternal life through the sacrifice of His Son Jesus the Christ, God’s anointed Son. The Messiah The King Of The Universe.
I will usually suggest the person read the book of St. John first. then read the four gospel accounts of Matthew, Mark, Luke and John again. Too, I suggest they pray and ask that God reveal Himself to them as they read. Then too, pray for them when ever the Holy Spirit puts them on your heart. God is more interested in getting them saved that you are. Pray that He put other labors besides you in their path. And believe that He will do it. Then trust that the Holy Spirit will do the creative work in their heart.
Lord, we pray for Randy that he be given creative ways to reach out to this individual. And that this persons mind be open to the gospel message. We pray for eyes to see, and ears to hear. And for a heart that understands. That the light dispels the darkness, and that they would come to recognize their personal need for the Savior.

We pray, in Jesus name. Amen! 💓