When people have things they want to remember, like a wedding invitation, or things they treasure, like a child's drawing, where do they put them? Many of them may use magnets to attach them to the refrigerator door. When employers have important notices or memos for their employees to see or information that may be of interest to them, where do they place them? Many times, these things will be tacked up on a bulletin board.
When we, as believers, give our hearts to Christ, we hand over our sinful nature and our worldly passions and desires to Him. But we don't just hand them over. In his letter to the Galatians, Paul wrote, "Those who belong to Christ Jesus have nailed the passions and desires of their sinful nature to His cross and crucified them there." (Galatians 5:24, NLT) We nail our sinful nature to the cross of Jesus Christ, the cross upon which He gave His life so that those sins may be forgiven.
The cross of Christ, where our worldly passions and desires have been nailed, becomes a reminder for us. Just like the bulletin board full of notices and memos, just like the refrigerator door full of notes and children's drawings, the cross serves a reminder that Jesus came to this earth and, in obedience to the Father's will, gave His life for us so that we may be free from our sins and our sinful nature and have eternal life with God.