True repentance: “Repent ye and believe the Gospel” (Mark 1:15)
“Testifying both to the Jews and also to the Greeks repentance toward God and faith toward our Lord Jesus Christ” (Acts 20:21).
A contrite spirit and a heart acceptance of the Gospel are inseparably connected, so that wherever the one is mentioned the other is presupposed. For example, take the passages recording the Gospel commission: in Mark 16:16 the emphasis is on “believing”, while in Luke 24:47 it is on “repentance”—the two together explaining the “make disciples” of Matthew 28:19. The one cannot exist without the other: it is just as morally impossible for an impenitent heart to believe, as it is for an unbeliever to repent. There may indeed be a mental assent to the Truth unaccompanied by any brokenness of heart, as there may be natural remorse where no faith exists; but there can be no saving faith where evangelical repentance is absent…There can be no pardon while there is no repentance (Isa. 55:7, Luke 24:47, Acts 3:19) i.e. mourning over and abandoning of our evil ways.
True Repentance is a Change of Mind
Repentance is a change of mind: one that goes much deeper and includes far more than a mere change of opinion or creed. It is a changed mind, a new perception, an altogether different outlook on things as they previously appeared. It is the necessary effect of a new heart. Repentance consists of a radical change of mind about God, about sin, about self, about the world. Previously God was resisted, now He is owned as our rightful Lord. Previously sin was delighted in, but now it is hated and mourned over. Previously self was esteemed, but now it is abhorred. Previously we were of the world and its friendship was sought and prized, now our hearts have been divorced from the world and we regard it as an enemy. Everything is viewed with other eyes than formerly, and an entirely different estimate is formed of them. The impenitent see in Christ no beauty that they should desire Him, but a broken and contrite heart perceives that He is perfectly suited to him. Thus, while He continues to be despised by the self-righteous Pharisees, He is welcomed and entertained by publicans and sinners. Repentance softens the hard soil of the soul and makes it receptive to the Gospel Seed.
True repentance necessarily leads to a change of conduct, for a change of mind must produce a change of action.
Source: Gleanings in Joshua by Arthur W. Pink