What is the Holy Spirit's role at salvation and why?

At salvation, the Holy Spirit seals believers and serves as God's earnest or down payment of the purchased possession. After belief, God seals us with the Holy Spirit of promise, guaranteeing the future redemption of the body, which will redound to the praise of His glory.

Les teaches that after a person hears the gospel—that Christ died and rose from the dead—the only requirement is to believe. After believing, God seals the believer with the Holy Spirit of promise. That Spirit is the earnest, the down payment. Salvation is complete for the soul and spirit: we are redeemed, justified, sanctified, and in the body. But the body is the purchased possession that is only partial; it must be ready for eternity. The Holy Spirit as God's down payment assures that purchase, and it will all redound to the praise of His glory.

Ephesians 1:14

Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.

Why does the Holy Spirit come in at salvation as a down payment?

At salvation the Holy Spirit comes in as God's seal and earnest, a down payment of the purchased possession. Salvation is complete for the soul and spirit, but the body is the third part still awaiting eternal readiness. The Spirit is God's pledge of that future completion, all to the praise of His glory.