If the Christian is saved by GRACE through faith in Messiah, why must they seek forgiveness, repentance, of sin subsequent salvation?

Christian response,

GRACE is NOT a license to sin void repercussions…

The one who has trusted in Jesus as Messiah by faith, having established New Covenant relationship with the Father and received the Holy Spirit as Guarantor of that Covenant (John 3; Ephesians 1:13-14), these are declared “not guilty” by the Father via covenant promise, forever (Hebrews 8:12).

The Christian is covered – protected – sealed in the imputed-gifted righteousness of Jesus by faith (2 Corinthians 5:21) yet the Adamic sin-nature (Romans 5:12) struggles with the new-nature in Christ (2 Corinthians 5:17; Romans 7:14-25); therefore, when a Christian sins in word, thought or deed, they remain forgiven by grace but the Christian is given 1 John 1:9 for the sake of conscience and intimacy with our Lord.

Our misbehavior…our temporary rebellion or mistake or error in judgment does NOT threaten eternal covenant with the Father but it does weaken intimacy as the Holy Spirit is grieved within us and our conscience is likewise grieved as the Holy Spirit employs, makes applicable, the Spiritual Law via the conscience (Romans 2:14-15); therefore, the Christian apologizes to the Father for their indiscretion and thereby reestablishes “intimacy” and “peace” within the inner man as intimacy with our Lord is the most cherished and fulfilling gift provided by the Holy Spirit (Romans 5:1).

The Law of the Spirit of Life v. the law of sin and death,

This is but one-reason why the “Law of the Spirit of Life” in the New Covenant is infinitely superior to the law of sin and death, the written code (Romans 8:2), as the indwelling Holy Spirit, in the New Covenant, methodically works through the redeemed conscience of the faithful in Messiah (2 Corinthians 5:17) and meticulously discerns applied discretion within the redeemed relevant to every moral decision encountered daily, a divine discernment that is not subject to rationalization, obfuscation, justification by the flesh; whereas, the written code is easily rationalized away by the sin-nature and a seared conscience, a compromise with the World that culminates in the death of mind, body, soul, eternally.