Marcus Caelius Rufus was born in 88 or 87 BC, at Interamnia in Picenum, where his father was categorised as a member of the "eques" (knight) class, a wealthy middle class placed just below the patrician upper class. From 73 to 63 BC, Caelius served a political apprenticeship under Crassus and Cicero. Throughout that apprenticeship, he became familiar with life in the Roman Forum. In 63 BC, Caelius turned his back on Cicero to support Catiline, who was running for consulship. It is unclear whether or not Caelius supported Catiline after the latter had lost the election and taken up arms, but he was not among the people prosecuted for their involvement in the conspiracy.