Students of God's Word understand Grace to be the unearned, or unmerited favor of God toward sinful man. The Scriptures teach us that we are sinners worthy of death, that we cannot do enough religious works to make up for our sin, and that it is only by trusting in the finished work of Christ, on our behalf, that we can be saved from the penalty of our sin (Rom. 3:23; 5:8; 6:23; Gal. 3:1-3; Eph. 2:8-9; etc.). Ephesians 2:8-9 says, "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: not of works, lest any man should boast."
Over the years, I have heard grace expressed as God's Riches At Christ's Expense. God, motivated by love for fallen mankind, sent His Son to pay for our sin so that we could become children of God (Jn. 3:16; 1:11-12; Lk. 2:36; Rom. 8:14-16; Gal. 3:26; 1 Jn. 3:1-2; etc.). As God's adopted children, we have become joint heirs with Christ; all that belongs to Him will one day belong to us (Rom. 1:15; Gal. 4:5-7; Eph. 1:5; Titus 3:7; Jam. 2:5; etc.). Do we deserve anything but the wrath of God? NO! But God loves us and gives us what we do not deserve; that is grace! By the way, mercy is not getting what we do deserve!
Do you know the best way to tell if someone is a genuine born again believer? While many would say it is the Fruit of the Spirit, because Jesus said, "By their fruit ye shall know them" (Gal. 5:22-23; Mt. 7:16, 20), the "fruit" is easy to fake, and the Lord's comment had to do with identifying false teachers. In my experience, the best way to identify a brother or sister in Christ is by their humility. A genuine believer realizes that their relationship to the Father is 100% the result of His grace!
The mark of a true believer is a grateful, humble heart!