Yep, that is exactly what Calvinism teaches, you can't lose your salvation, they own this doctrine.
The Holy Spirit is your down payment on your salvation until the resurrection.
Eph. 1:13-14
"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,
Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory."
You are not made a purchased possession of Christ until the resurrection.
You do greatly err, my friend!
@JesusFan
Hopefully this will help you to understand.
God saves man body, soul and spirit.
If man holds to the end his faith in Christ, his body goes to the grave, the soul and spirit go to be with Him by the gift of the indwelling Holy Spirit through faith. The gift of the indwelling Holy Spirit is as Paul said, it is a down payment on your salvation through your faith.
The body is not yet saved, that is yet to come at the resurrection. That is when you are a complete purchased possession of Christ.
As the Scripture says, "we are made partakers of Christ if we hold the beginning of our conversation (our profession of faith) to the end."