John the Baptist prepared the way for Christ by preaching repentance (Mk 1:4); Christ Himself preached the same (Mk 1:14-15) and so did His disciples. (Mk 6:12) Repent: this is Christ’s first call.
To repent is to change your mind, to start thinking differently, and the context here is sin (Mk 2:17), which is breaking God’s Law. (1Jn 3:4) God is introducing Himself by saying, Change your mind about breaking My Law.
This isn’t quite the same as, stop sinning; no one can totally stop sinning and live perfectly. (1Jn 1:8) It’s more like … stop sinning on purpose, deliberately, intentionally; stop thinking it’s OK to sin, that God doesn’t mind.
Sin is offensive to God, and willful, intentional sin angers Him. (He 10:26-27) Choosing sin is choosing darkness (Jn 3:19), choosing the lie; and God is light (1Jn 1:5); God is Truth. (Jn 14:6) Walk in the light. Pursue the truth. Do your best to obey God’s Law, all of it, as well as you can, and keep asking Him for help where you’re still failing to keep it perfectly. It’s the only way to be in relationship with God. (1Jn 1:6-7)
Christ follows this call to repentance with a call to believe the gospel, the good news that the kingdom of God is open to us. He doesn’t start with this message; that would be like the King giving us directions to His home while we’re still defying Him and running away; it doesn’t even make sense. Before giving us directions to help us find the Way, we must be seeking Him. (He 11:6)
Those who aren’t trying to obey God don’t know Him (1Jn 3:6); those who intend to continue offending Him have alienated themselves from salvation itself. (Ps 119:155)
Salvation isn’t so much about deliverance from Hell as it is the offer of a new nature that’s inclined to obey God’s Law (He 8:10), freeing us from the power and dominion of sin so we can fellowship with Him. (Ro 6:22) Repentance is God’s gift (1Ti 2:25), opening the door to salvation, enabling us to turn from death to life. (Ac 11:18)
This may explain why Christ replied to the rich young ruler the way He did (Mk 10:17-19); it was an invitation to take God’s Law seriously. The Law is our teacher to bring us to Christ (Ga 3:24); until we earnestly submit to this divine teacher and learn from Him we won’t find Christ.
To profess Christ yet not do what He says (Lk 6:46) is to deceive ourselves (Ja 1:22), miss Heaven altogether (Mt 7:21), and store up eternal wrath for ourselves. (Ro 2:8-9)