The book of Romans is a great exposition of what grace means. God has justified us by grace (see e.g., Romans 3:24). He has accepted us in Christ. In Romans 14 (and Romans 15:1–33) this principle of acceptance by grace is applied to the mutual relations in the church. God has accepted us and therefore we shall accept each other. (Romans 14:1 and Romans 15:7 is an inclusio, which is a concentric principle by placing similar material at the beginning and end of a section.)
1 As for the one who is weak in faith, welcome him, but not to quarrel over opinions.