Generally, atheists consider it likely that a person, now known as Jesus of Nazareth, existed, but they don’t believe that he was the son of a god, nor do they believe he performed miracles.
The question ‘Do atheists believe in Jesus?’ could mean several things. It could mean ‘Do atheists believe that a person called Jesus existed?’, or it could mean ‘Do atheists believe that Jesus was really the son of god?’, or it could mean the more abstract ‘Do atheists have trust in the supernatural being called Jesus?’.
Generally, atheists think it likely that a person existed who we nowadays refer to as Jesus of Nazareth. A mythology developed around this person, which is nowadays called Christianity, but it’s just that – a mythology. Some of the stories about Jesus of Nazareth might be partly or even mostly true; some of them are likely not true at all. But crucially, atheists don’t believe that Jesus of Nazareth had any supernatural qualities or powers. He did not perform miracles, because he (if he existed) was just an ordinary person.
So atheists think that it’s likely that the historical figure of Jesus did exist, but he could not perform miracles, he did not come back to life, and he was not the son of a god.