Doubts about the existence of an historical Jesus are nothing new. We can read of doubts in The Bible about an historical existence of Jesus in the book of 2 John, written late in the 1st century or early 2nd century.
John 2 tells of many preaching about Jesus Christ at the time but not considering him as having existed in flesh and blood:
2 John 1:
7 Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist. 8 Watch out that you do not lose what you have worked for, but that you may be rewarded fully. 9 Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son. 10 If anyone comes to you and does not bring this teaching, do not take him into your house or welcome him. 11Anyone who welcomes him shares in his wicked work.
1 John is much the same:
1 John 4:
1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. 2 This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, 3 but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.