Jesus is Risen
All of Christianity rests on the resurrection of Jesus from the dead. If He is indeed risen, then He is indeed the Christ, the Son of God. From this, it follows that the Christian God is, in fact, the true God.
Paul affirmed this in his opening statement to the Romans:
Paul, a bond-servant of Christ Jesus, called as an apostle, set apart for the gospel of God, 2 which He promised beforehand through His prophets in the holy Scriptures, 3 concerning His Son, who was born of a descendant of David according to the flesh, 4 who was declared the Son of God with power by the resurrection from the dead, according to the Spirit of holiness, Jesus Christ our Lord. Romans 1:1-4 NASB
The resurrection of Jesus from the dead powerfully declares that He is the Son of God, the Christ, and our Lord.
What flows from this is forgiveness, redemption, belonging, victory over death, victory over sin, and a host of other things.
Everything else--fulfilled prophecy, miracles, answered prayer, transformed lives--is powerful, but secondary to the resurrection of Jesus.
If Jesus is not risen, Christianity would not exist.
If Jesus is risen, Christianity is true.