Mark 1:15

And saying, the time is fulfilled
Either that which was fixed for the end of the law and prophets, the legal and Mosaic dispensation, and the Jewish church state; or the fulness of time for the Messiah's appearance in the world; which was agreed upon between the Father and the Son, was predicted in various prophecies, and the people of the Jews were in a general expectation of:

and the kingdom of God is at hand:
the same with the kingdom of heaven, in ( Matthew 3:2 ) ( 4:17 ) , see the notes: (See Gill on Matthew 3:2), (See Gill on Matthew 4:17).

repent ye, and believe the Gospel.
He called them to repent, not only of their former sins and vicious course of life, but of their bad principles and tenets, concerning a temporal kingdom of the Messiah; concerning merit and free will, justification by the works of the law, and salvation by their obedience to the ceremonies of it, and the traditions of the elders: these he exhorts them to change their sentiments about, and to relinquish them, and give into the Gospel scheme; which proclaims liberty from the law, peace, pardon, and righteousness by Christ, and salvation and eternal life by the free grace of God.