How do you explain this?

You can't look at single verses in the Bible and expect them to make sense standing alone. In this verse, Paul was writing to the church in Corinth... perhaps a different version of the Bible might be more useful in trying to understand what is being said: "God affirms us, making us a sure thing in Christ, putting his Yes within us. By his Spirit he has stamped us with his eternal pledge—a sure beginning of what he is destined to complete." (The Message) What I think it basically means is that God has put time and love into us as a "deposit" to show He will show through for us. Just like someone puts down a deposit for something expensive to show that they really are serious about purchasing it - so God has done for our souls.