As a believer, and someone who grew up in church, it seems like it's common knowledge that Christ died on the cross for our sins. When I heard the truth of the gospel of salvation I believed and was saved from my sins. It's hard to remember there is a majority of people out there who reject God and salvation. It becomes easy for me to think that I'm the smarter person, not remembering that it was God who called me to salvation.
With that salvation, God had to seal it with the Holy Spirit. Why? Just as we wouldn't have made the choice to believe on our own, we wouldn't make the choice not to sin on our own. When left to my own decision making, all to often I choose sin over what God would have me do. Thankfully, Ephesians 1:13 tells of God's promise to seal our salvation with the Holy Spirit. Without that seal, I think the Arminians and Nazerenes, some of who believe you can lose your salvation, might have some weight to that belief. If salvation was based on our own ability to save ourselves with the good we do, we'd all be in a really bad place and headed to a worse one. Without God's calling, our selfish desires would stop us from achieving any good. I can't imagine life without God's common grace (grace shown to all mankind), let alone His saving grace.
Ephesians goes on in verse 14 to tells us our inheritance is guaranteed by the Holy Spirit. He is "the guarantee of our inheritance until we acquire possession of it," or until we are in heaven with God. Without the Holy Spirit we wouldn't make the good decisions in our lives. We wouldn't think, "I'm in sin," or "I'm about to violate God's laws," and flee from that decision. Only by God's good grace are we given a helper to guide us to those right choices, all to the glory of God.
Praise God for His Guidance.