Some people tend to self-identify in ways that are not according to dictionary definitions. Pew Research Center is a respected surveying organization that uses established protocols to determine the status and opinions of various populations. The link in my source area featured a number of interesting results from a 2016 survey that would apply to your question:
For instance, 8% of those surveyed who self-identified as atheists also said that they believe in a God or a universal spirit.
Also, some people who self-identified as a member of a religious group said that they did not believe in God.
If one defines Christian according to what the Bible and Jesus Christ said what a Christian is, these scriptures can provide some insight. After reading them, you decide if according to Jesus Christ you can be an atheist and a Christian:
16 “For God loved the world so much that he gave his only-begotten Son, so that everyone exercising faith in him might not be destroyed but have everlasting life.
33 Whoever has accepted his witness has put his seal to it that God is true.
36 The one who exercises faith in the Son has everlasting life; the one who disobeys the Son will not see life, but the wrath of God remains upon him.
39 “You are searching the Scriptures because you think that you will have everlasting life by means of them, and these are the very ones that bear witness about me. 40 And yet you do not want to come to me so that you may have life. 41 I do not accept glory from men, 42 but I well know that you do not have the love of God in you.
John 8:31, 32
31 Then Jesus went on to say to the Jews who had believed him: “If you remain in my word, you are really my disciples, 32 and you will know the truth, and the truth will set you free.”
John 13:34, 35
34 I am giving you a new commandment, that you love one another; just as I have loved you, you also love one another. 35 By this all will know that you are my disciples—if you have love among yourselves.”
12 Most truly I say to you, whoever exercises faith in me will also do the works that I do; and he will do works greater than these, because I am going my way to the Father.
3 This means everlasting life, their coming to know you, the only true God, and the one whom you sent, Jesus Christ.
31 But these have been written down so that you may believe that Jesus is the Christ, the Son of God, and because of believing, you may have life by means of his name.
8 But as for the cowards and those without faith and those who are disgusting in their filth and murderers and the sexually immoral and those practicing spiritism and idolaters and all the liars, their portion will be in the lake that burns with fire and sulfur. This means the second death.”