A person who has realized that they have sinned and fallen short of the glory of God, and has humbly turned from their sins(the definition of repentance), and has submitted their life to the Lordship of Jesus Christ and has trusted in Him(Jesus Christ) as Lord and Savior and obey Him! Notice, I say trust in Jesus Christ, the son of God alone! We don't trust in "mother" Mary, saints, a priest, the pope,
the book of mormon, or anything like that.
A Christian is not someone who is merely religious, goes to church, prays prayers, reads bibles, was baptized, or any of that stuff. That stuff is good, but if there's no inward change that results in outward change and good fruit, they are not truly saved.
In addition, despite what many say, God's Grace is NOT a license or a covering for sin, and Christians are not unconditionally eternally secure. A Christian can lose their salvation by renouncing Jesus Christ, especially during times of persecution, by unrepentant willful sin, or by believing a false gospel, or by taking
the Mark of the Beast during the 7 year Tribulation period when the Antichrist will be reigning and will require people to worship him and take a mark of loyalty indicating their loyalty to him(the Antichrist) in order to partake in commerce as well to avoid beheading. However, the Bible warns that if ANYONE takes the mark, regardless of the reason, they will spend eternity in the Lake of Fire. Taking 666 is an
unforgivable sin.
Revelation 14:9-11.
The twelve Apostles, the Apostle Paul, those who have died as martyrs for the faith, the future Tribulation Christians who endure to the end by refusing to take the 666 mark despite
the pressures of
social rejection, persecution, possible torture, starvation, and
the death penalty.