I gather by this that you think repentance is a one time thing? The Greek μετανοειτε in Mark 1:15 is in the present tense. This Greek tense is use in both the senses of the English simple present and present progressive. So I would render it a imperative present progressive: Be repenting
Unless I am mistaken then, clearly with the belief that you need repent only once you believe that God has a righteous vision of you and you need do nothing further to be righteous.
“I urge you, brothers, to watch out for those who cause divisions and put obstacles in your way that are contrary to the teaching you have learned. Keep away from them.”
Apostle Paul to the Romans
Tis a one time thing: turning going to other way. change of heart or mind that leads to change of direction. Believe means trust in rely on. Gospel is Jesus came as God in the flesh to die for sin and rose again that we might have life in him.
Tense: Present
Voice: Active
Mood: Imperative
1) to change one’s mind, i.e. to repent
2) to change one’s mind for better, heartily to amend with abhorrence of one’s past sins
“Repentance (metanoia, ‘change of mind’) involves a turning with contrition from sin to God; the repentant sinner is in the proper condition to accept the divine forgiveness.” (F. F. Bruce. The Acts of the Apostles [Greek Text Commentary], London: Tyndale, 1952, p. 97.)