We are justified not because we repent and believe (we should not separate these two things) but because Christ died. However the instrument through which we receive the benefits of Christ's death is through faith and repentance and so without faith and repentance we will not be saved.
Concerning repentance, "the Jews commonly express it by
hbwvt a "turning", or "returning", and it is frequently signified in the Old Testament by a man’s turning from his evil ways, and returning to the Lord; the term from which he turns is sin, the term to which be turns is the Lord, against whom he has sinned; and what most powerfully moves, encourages, and induces him to turn, is the pardoning grace and mercy of God through Christ (Isa. 55:7), and so in the New Testament, repentance and turning are mentioned together, and the latter as explanative of the former; (see Acts 3:19; 26:20)..."
"...the Greek word more frequently used in the New Testament for repentance is
metanoia, which signifies an "after understanding", or "after knowledge"; as when a man takes into serious consideration a fact after it is committed, and thinks otherwise of it, and wishes he had not done it, is sorry for it, and resolves, through the grace of God, to forsake such practices; this is a proof of a man’s wisdom and understanding; now he begins to be wise, and to show himself an understanding man; even an heathen could say, "Repentance is the beginning of wisdom, and an avoiding of foolish works and words, and the first preparation to a life not to be repented of." It is a change of the mind for the better, and which produces change of action and conduct: this, as it is expressive of true repentance, flows from the understanding being enlightened by the Spirit of God, when the sinner beholds sin in another light it did, even as exceeding sinful; and loathes it, and abhors it and himself for it. There is another word the Greeks use for repentance,
metameleia, and though the noun is not used in the New Testament, the verb is (Matthew 21:29,32), and signifies a care and anxiety of mind after a fact is committed, a concern with sorrow that it should be done, and a care for the future not to do it again; hence the apostle, among the genuine fruits of godly sorrow for sin, mentions this in the first place, "What carefulness it wrought in you", not to offend more (2 Cor. 7:11). It also signifies a change of mind and conduct, as appears from (Matthew 21:29), a penitent sinner has another notion of sin than he had; before it was a sweet morsel, now a bitter and evil thing; before his heart was bent upon it, now determined through divine grace to forsake it, and cleave to the Lord with full purpose of heart."
Furthermore:
"There is an evangelical repentance, which lies,
2e2a. In a true sight and sense of sits; in a sight of it, as in itself considered as exceeding sinful in its own nature, and not merely as in its effects and consequences ruinous and destructive; not only in a sight of it in the glass of the divine law, but as that is held in the hand, and seen in the light of the blessed Spirit; and in a sight of it as contrary to the pure and holy nature of God, as well as repugnant to his will, and a breach of his law; and in a view of it as it appears in the glass of pardoning love and grace.
2e2b. In a hearty and unfeigned sorrow for it; this sorrow for it is the rather because it is against God, and that not only as a holy and righteous Being, but as good, and gracious, and merciful, of whose goodness, both in providence and grace, the sinner is sensible; the consideration of which increases his sorrow, and makes it the more intense and hearty.
2e2c. It is attended with shame and confusion of face, as in Ezra 9:6,8,10 and Luke 18:13 this shame increases the more, the more a sinner is sensible that God is "pacified towards him for all that he has done" (Ezek. 16:63).
2e2d. Such a repentance is accompanied with a loathing, detestation, and abhorrence of sin as the worst of evils; to truly penitent sinners sin appears most odious and loathsome; nay they not only loath their sins but themselves for them, and the rather when most sensible of the goodness of God in bestowing both temporal and spiritual blessings on them, and especially the latter (Ezek. 20:40-44; 36:25-31), yea they abhor it as of all things the most detestable, when they are in the exercise of this grace; so it was with holy Job, when favored with a special sight of the greatness and goodness of God (Job 42:6; Isa. 6:5).
2e2e. Where this repentance is there is an ingenuous acknowledgment of sin, as may be seen in David (Ps. 32:5 51:3 in Dan. 9:4,5), and in the apostle Paul (1 Tim. 1:13-15), so the prodigal, as soon as he came to himself, and was made sensible of his sin, and repented of it, went to his father, and said to him, "Father, I have sinned against heaven and in thy sight" (Luke 15:21), and to encourage such a sincere repentance and ingenuous confession, the apostle John says, "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness" (1 John 1:9).
2e2f. It is followed with a resolution, through the grace of God, to forsake sin; this the sinner is encouraged unto, as before observed, by the abundance of pardon through the mercy of God in Christ (Isa. 55:7), and indeed it is only such who can expect to share in it; "Whoso confesseth (sins) and forsaketh them, shall have merry" (Prov. 28:13). Now such a repentance appears to be evangelical; inasmuch, as
2e2f1. It is from the Spirit of God, who reproves for sin and convinces of it, enlightens the eyes of the understanding to see the exceeding sinfulness of sin; and as a Spirit of grace and supplication works this grace in the heart, and draws it forth into exercise, to mourn over sin in a gospel manner at the throne of grace (Zech. 12:10).
2e2f2. Such repentance, in the exercise of it, follows upon real conversion and divine instruction, "Surely after that I was turned I repented, and after that I was instructed, I smote upon my thigh" (Jer. 31:19) upon such a turn as is made by powerful and efficacious grace, and upon such instruction as leads into the true nature of sin, the effect of which is blushing shame and confusion.
2e2f3. Is what is encouraged and influenced by gospel promises, such as these in (Isa. 55:7; Jer. 3:12,13), now when repentance proceeds not upon the terrors of the law, but upon such gracious promises as these, it may be called evangelical.
2e2f4. It is that which is attended with faith and hope: repentance towards God, and faith in our Lord Jesus Christ, go together as doctrines, and so they do as graces; which is first in exercise is not easy to say; our Lord says of the Pharisees, that they "repented not, that they might believe", which looks as if repentance was before and in order to believing (Matthew 21:32), and elsewhere faith is represented as first looking to Christ, and then repentance or mourning for sin; repentance, as some have expressed it, is a tear that drops from faith’s eye (Zech. 12:10). However, that is truly evangelical repentance which has with it faith in the blood of Christ for the remission of sins; for repentance and remission of sins, as they go together as doctrines, so also as blessings of grace (Luke 24:47; Acts 5:31), for where true repentance for sin is, there must be faith in Christ for the remission of it, at least hope of pardon by his blood, or else such repentance would issue in despair, and appear to be no other than the sorrow of the world which worketh death.
2e2f5. It is such a repentance which flows not from dread of punishment, and from fear of the wrath of God, but from love to God, and of righteousness and holiness, and from an hatred of sin; they that love the Lord hate evil, and they love righteousness and hate evil because he does; and when tempted to sin reason after this manner, "How can I do this great wickedness, and sin against God", so holy just, and good, and who has shown such love and kindness to me? (Gen. 39:9), it was love to Christ, flowing from a sense of pardoning grace and mercy, that fetched such a flood of tears from the eyes of the penitent woman at Christ’s feet, with which she washed them, and wiped them with the hairs of her head; and which caused Peter, under a sense of sin, to go out and weep bitterly (Luke 7:37,38,47; 22:61,62)."
See:
Of Repentance Towards God