2 Corinthians 1:21-22 (NASB)
"Now He who establishes us with you in Christ and anointed us is God, who also sealed us and gave us the Spirit in our hearts as a pledge."

The Holy Spirit of promise makes Christians firm in the faith of the Gospel, allowing the believer to live a life pleasing to God, and providing assurance of salvation.

"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, who 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." (Ephesians 1:13-14)

God has made known to believers through Christ, the Spirit of truth, this promise of salvation. Through Christ, God and humanity formerly estranged, are now reconciled; and the penalty of sin is satisfied for the wrong which caused the separation. He brought about, by His Spirit, the benefits of faith and love, whereby we are united with God. The sanctifying and comforting influences of the Holy Spirit seal or designate believers as the children of God, and the beneficiaries of eternal life. All to the glory of God.

"And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption." (Ephesians 4:30)

Anything that arouses evil desires and lusts grieve the Spirit of God. Corrupt passions such as bitterness, rage, anger, a complaining attitude, slander, and malice sadden the Holy Spirit. The continuation of these things in one's life causes the Spirit of God to withdraw His presence and His sanctifying influences. When the believer has the Holy Spirit living within, that person can truly be kind, compassionate, forgiving, and bear good fruit (Galatians 5:22-23).