What happened when you 1st believed in Christ?
Do you remember buying your first home? After searching you decided this is the house. You met with the realtor who asked for a deposit or down payment. That payment is a guarantee that you are serious and committed to the full purchase.
In the same way when you became a Christian Paul tells us in Ephesians 1:13 “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.”
You are now completely secure in Christ!