Their teachings that man is sinful and worthy of death and in need of a savior to rescue him and free him of his sinful tendencies or else feel the wrath of God.
This is only one of the iterations of Christianity. It's not a biblical teaching, nor will you see any of the apostles that taught it. John wrote:
2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3 And every man that hath this hope in him purifieth himself, even as he is pure.
4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
5 And ye know that he was manifested to take away our sins; and in him is no sin.
1 John 3:2-5
Man is not "deserving" of Hell. Christianity teaches that through Adam all men fell. But through Christ, all men would be made alive. In other words, man is created with a type of safety net. Without Christ, we would indeed be lost, without God and without a place in His creations. Christ, however, saves even the worst sinner after he has paid the price of his own sins.
Many Christians are speaking of man's fate without Christ and they assume sinners don't have that safety net. But yes, part of the reason we're placed on Earth is to combat our own natural tendencies to do evil. Through Christ, man achieves the ability to become like God.