St. Paul had a strange title for the Holy Spirit. He called the Spirit "the Down Payment." The Lord has promised us new life, freedom, joy, peace, love, power, authority, healing, victory, communion with Him, forgiveness of sins, resurrection from death, and eternal life. He has promised us even much more, but these are just a few highlights.
How do we know these promises are true? Primarily we know this because Jesus said so, and His word is true. He also lets us personally experience some of what He's promised. He does this through the power of the Holy Spirit. The Spirit is our Down Payment. For example, because the Spirit healed me or healed through me, I'm sure that nothing can separate me from the love of Christ (Rm 8:35). Because the Spirit has given me the gift to praise the Lord in another language, I know the Bible is true. Because the Spirit gave me peace in the midst of the storm, I know I'm going to heaven.
The Spirit is a Down Payment that confirms us. Even in a confused, doubting, secular society, we're confirmed that the Lord will fulfill all His promises because the work of the Holy Spirit is "the first Payment" (Eph 1:14).