Those under the Kingdom Gospel could lose their salvation. Not so for those in the Age of the Gospel of Grace - the mystery revealed to Apostle Paul.
In Acts 5:3 Ananias (& his wife) committed the unpardonable sin - they blashpemed the Holy Ghost and immediately lost their salvation. They were clearly believers, as theHoly Ghost was in them and they had to give it up after they lied to the Holy Ghost. This clearly does not happen in this dispensation of the Age of Grace, but will be possible again when the Church is raptured and the prophetic Kingdom of God program resumes (see Hebrews - a letter addressed to Hebrews, not Gentiles, who were preparing for the tribulation period to come. ) And Luke for info on how those in the Kingdom Gospel can lose their forgiveness.
Hebrews 6:4-6
(KJV Strong's)
4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
Luke 12:10
(KJV Strong's)
10 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven.
Matthew 12:31-32
(KJV Strong's)
31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. 32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.
BUT WE, THE BODY OF CHRIST, ARE SEALED BY THE SPIRIT. ALL OF OUR SINS ARE FORGIVEN NOW - WE ARE NO LONGER BEING JUDGED FOR SIN. CHRIST PAID THAT PRICE IN FULL ALREADY.
The good news for us:
Ephesians 1:13
In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were SEALED with that holy Spirit of promise,
2 Timothy 2:19
Nevertheless the foundation of God standeth sure, having this SEAL, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity.