Simple.
If you fall away in unbelief after repenting, and God tuns you over to that unbelief, you are not allowed to come to repentance again to taste the heavenly gift and share in the Holy Spirit and experience the powers of the age to come.
We know from the whole counsel of scripture that God may make an attempt to bring the ex-believer back to faith in Christ after they leave (i.e. the Galatians) but if they don't come back they will eventually be locked out from being able to come back.
So keep believing. And if you have already fallen away in unbelief come back while you still can. Don't play games.
"24 As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides (continues) in you, you also will abide (continue) in the Son and in the Father."-1 John 2:24
"9 Anyone who goes too far (casts the word of God behind them) and does not abide (continue) in the teaching of Christ, does not have God; the one who abides (continues) in the teaching, he has both the Father and the Son."-2 John 1:9
"God has given us eternal life, and this life is in His Son. 12 He who has the Son has the life; he who does not have the Son of God does not have the life."-1 John 5:11-12
"they were broken off for their unbelief, but you stand by your faith. Do not be conceited, but fear; 21 for if God did not spare the natural branches, He will not spare you, either.22 Behold then the kindness and severity of God; to those who fell, severity, but to you, God’s kindness, if you continue in His kindness; otherwise you also will be cut off."-Romans 11:20-22
The message is plain and it is clear: Stay in Christ by continuing to trust in him or you will not continue in the Father and the Son who is eternal life. If you leave and you resist all efforts of God to bring you back to faith in Christ you will eventually not be
allowed to come back to repentance again.