Eph 1:13 says: In Him, you also, after listening to the message of truth, the gospel of your salvation-- having also believed, you were sealed in Him with the Holy Spirit of promise. The word for "sealed" is σφραγίζω (sphragizo) which "signifies ownership and the full security carried by the backing (full authority) of the owner. "Sealing" in the ancient world served as a "legal signature" which guaranteed the promise (contents) of what was sealed." (Helps Word Studies).
By the sealing ministry of God the Holy Spirit, God has guaranteed our eternal secrity. Furthermore both the Father and the Son participate in the guarantee in John 10:28-29. In this passage it says we shall "never ever" perish. In the Greek this is οὐ μὴ ἀπόλωνται (ou me apolontai). οὐ (ou) and μὴ (me) are two different words for "not" meaning "never ever" and ἀπόλωνται (apolontai) is from ἀπόλλυμι (apolummi) which is the exact same word used in John 3:16: For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish (ἀπόλλυμι (apolummi), but have eternal life.
There is punishment for turning away from God once saved, but this does NOT include loss of salavtion. This is also made clear in the parables and metaphors of scripture. In the parable of the Prodigal Son (Luke 15:11-32) the wayward son never ceased being the Father's son. Once a son, always a son. Also, as believers, we are said to be "born again" (John 3:3). It is impossible to be "unborn." Being born signifies a one-time event that cannot be undone.
