At the moment of salvation we're sealed by the Holy Spirit.

(Ephesians 4:30 NIV) And do not grieve the Holy Spirit of God, with whom you were sealed for the day of redemption.

It's impossible for those who are truly saved to lose their salvation. John makes it clear that those who abandon the faith were never saved in the first place.

(1 John 2:19 NIV) They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.