YouVersion

Stated herein, salvation is an ongoing process through holding fast to the gospel of JC. The Bible clearly teaches that we are sealed by the Holy Spirit (Eph 1:13, 4:30 etc) and assurred of our salvation (1 John 5:13, 1 Thess 1:5, etc.), but there's no such thing as sliding easily into the Kingdom i.e. christianity of your own accord. In the military, when the drill instructor gives the command, "Hold Fast!," the unit he's commanding knows to continue the current action without deviation. That's precisely what Paul is saying: continue precisely in the gospel that you've received
that you may continually be saved and look more like Christ.