To believe in the Lord Jesus means that you acknowledge that He’s the only begotten Son of God (or God in human form if you understand the Trinity). That He came in the flesh to live a sinless life, die sacrificially for our sins on the cross, and resurrect after 3 days and is soon coming back to claim His church.
Heartfelt devotion is where you realize your own sinfulness (His atoning sacrifice doesn’t make any sense if you don’t realize that) and repent of your sins and commit your life to following Him as Lord and Saviour.
To “believe” is to rely on, commit to, surrender to, or give your heart to. And through His resurrection you can experience a new life free from sin if you invite Him to be your personal Lord and Saviour.