To answer in the simplest form: Jesus Christ - The Son of the Living God. However, the subject is far more complex. How does one KNOW that they are in Christ? What Jesus Christ do they believe in?
Most people, in this world know of the "historical" Christ. Through stories or movies about His life. Even the Muslims recognize the historical Christ but their knowledge of Him is flawed. Many religionist today, teach a simplified Christ. Just walk the aisle and confess his name and be baptized. Often this is not genuine faith in the Savior but a temporary exercise of the flesh.
To know Christ is to know the Jesus Christ of the Bible. His Person and Work. His Divinity and Impeccable Humanity. He is the Word, the Great I Am, the Covenant Lord of Israel and the Lord of the New Covenant, which is in His Blood. To know one is in Christ, is to except everything said about Christ in all of Holy Scripture. If one rejects any part of Scripture about His Holy Character or His Work, then that one is none of His. (Rom. 8:9) Therefore, to have great assurance that one is in Christ, that one must study to learn all there is to know about Him.
For instance - a few questions:
1) Did Christ have his beginning from the virgin birth or from all eternity?
2) Is Christ the incarnate second Person of the Godhead?
3) Is Christ God?
4) Is Christ the Creator of all things?
5) Is Christ both fully God and fully man?
6) Is Christ's Nature's impeccable or peccable?
7) Did Christ die for everyone or the Sheep?
How one answers these questions will show whether they know the Christ of the Bible or some manmade perversion of Christ.
As Romans 8:9 indicated, one cannot know Jesus Christ if they have not the Spirit of Christ. When Adam fell and we all fell in him, mankind became spiritually "dead" or depraved. Everyone of us come into the world at enmity with God. We are sinners and love sin because our fallen nature loves sin. We cannot please God and have NO DESIRE to have God in our lives. Therefore, we cannot come to Christ in our fallen state because it would be against our nature.
John_6:44 No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.
John_6:65 And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
This being the case, every believer must be "Born Again". (John 3:1-10). In this new birth, a person is given a "new heart" (Ezek. 11:19; 36:26; Mat. 9:17), he becomes a "new man" (2 Cor. 5:17) and is given a God given "faith" (Eph. 2:8). This enables the person to move towards God and believe on Jesus Christ. (Thus, "given unto him of my Father", in John 6:44&65). This is the Sovereign Work of the Holy Spirit, who comes at the appointed time and dwells in the believer. Thus, having our nature changed, our will is changed towards the things of God and not away from them. Without these things, it is impossible to please God. (Rom. 8:8) Impossible to know God (Rom. 1:20-23; 1 Cor. 2:12, 14). Will not seek God. (Rom. 3:10-18)
Therefore, God - the Father - enables every single person, through the Holy Spirit, to come to the Biblical Christ and none other.
Having been "Regenerated", (New Birth), the believer begins his/her "Conversion" experience. With the aid of the Holy Spirit, who guides our thoughts and aids our prayers, we grow in Grace and knowledge of our Lord and day by day put on Christ and put off the "old man". We do battle with the flesh, which means we hate sin - especially our own sins. We fight for the Truth of the Gospel and all of God's revelation and will not tolerate any attempt to diminish the Person and Work of our dear Lord, Jesus Christ.
To many, today, want the benefits of Christ's Work in Salvation, or the power of the Spirit but they care not for the True Person of Jesus Christ. The Holy and Perfect, Impeccable God-Man. The Lamb of God, without Spot or Blemish.