The Revelation of Jesus Christ.
Note that it is not simply a revelation from Jesus Christ, but the "revelation of Jesus Christ." It was not simply that Christ told Paul something, but that Christ Himself revealed Himself to Paul, and in him, and He is the truth. That this is what is meant here may be seen from verse 16, where we read that God revealed His Son in Paul, that he might preach Him among the heathen. The mystery of the Gospel is Christ in the believer, the hope of glory. Col. 1:25-27.