"And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession—to the praise of his glory." (Ephesians 1:13-14)
The Greek word translated "deposit" (arrabon) speaks of a down payment that was made to secure a purchase. The moment you were saved, the Spirit of God became the down payment on God's final installment of your eternal glory. In modern Greek, arrabon includes the idea of an engagement ring, a deposit securing something in the future. The Holy Spirit is our guarantee that Christ will come again and take us with Him. In that sense, we can view the Holy Spirit as God's engagement ring to his Church, a sign of His deep love and His guarantee to keep His promises. Jesus and His Father give us a guarantee of His promise to marry us. If that marriage in Heaven does not happen between Him and us, then God did not honor His guarantee. So, if a person can lose their salvation, then God is a liar.
Christ purchased us. If He purchased us, He owns us, and we cannot get out of it. Therefore, a person cannot lose their salvation, because Christ bought them. He did not purchase every single person, for if He did, then every single human being would go to Heaven. He only purchased the Elect. And if He purchased us, then our belonging to Him has to do with Him, not with us. “It does not, therefore, depend on human desire or effort, but on God’s mercy.” (Romans 9:16)
Salvation is the work of God, not of man. It has to do with the sovereignty of God. The fact that salvation is the work of God, and not of man, works to the praise and glorification of God. But if it is up to man, then it works to the praise of man.