“And calling the crowd to himself.” This record is in Matthew 16:24-28; Mark 8:34-9:1; and Luke 9:23-27. Matthew says Jesus is speaking with his disciples, which was the major intent of what he was saying, however, Mark and Luke point out that the multitude was there also and was listening to this part of what he was teaching. What Jesus taught about him being the Messiah and suffering and dying was only taught to the disciples, which is why for this teaching he had to call the multitude to him.
“he must.” In this context, the Greek imperative verb is best translated “he must,” not “let him” (see commentary on Matt. 16:24).
“take up his cross.” The follower of Christ must be willing to suffer for Christ. [For more on the meaning of “take up his cross,” see commentary on Matt. 16:24].
“he must deny himself and must take up his cross, and then follow me.” The first two things, denying oneself and taking ones cross are aorist imperatives, while the “follow me” is a present imperative. So to be a true follower of Jesus Christ one must deny themself and take up their cross and then they are qualified to genuinely follow Jesus.