Accurate post Sunlover. I'm not even sure that a person today could commit this sin. It would require witnessing and denying the miracles the Holy Spirit worked through the Messiah. Christ as Israel's Messiah came unto his own and his own received him not.
Gurubu, if you have believed in Christ for salvation, you were sealed in him a that moment and the Holy Spirit was given you as a guarantee of your redemption. You are in union with Christ and he has said he will never leave or forsake you. There is no way you can damage your union with Christ because you are kept by the power of God, not your own power. (1 Pet. 1:5)
The seal of the Holy Spirit is from Eph 1:12-14. It is clear in the English but more so in the Greek.
"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, 14who is given as a pledge of our inheritance, with a view to the redemption of God's own possession, to the praise of His glory.
"having also believed" is an aorist, active, indicative. The aorist tense views the occurance of the event. Active voice, the person commits the event or action of believing. Indicative mood is a dogmatic statement of fact.
"you were sealed" is an aorist, passive, indicative. Upon the event of believing you were sealed. Passive voice, the action of sealing was preformed upon you at the event of your belief. Indicative mood, a dogmatic statement of fact. You were branded as God's possession.
My point is there is nothing you can do to unseal yourself and God will not.
38For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers,
39nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord.