Why should I believe that I cannot be separated from God's love?
Because Jesus said His sheep shall never perish and no one can pluck them out of His hand. Paul includes the whole sphere of creation—angelic and satanic powers—so nothing can separate His own from Him. If you believe God in Christ died and rose for your salvation, you should believe you are His.
Les teaches from John 10:27–28 that Jesus is speaking especially to the nation of Israel, using the term sheep: His sheep hear His voice, He knows them, and they follow Him. Jesus says He gives them eternal life, they shall never perish, and neither shall any pluck them out of His hand. Les points out the translators added “man” in verse 28, and he prefers to leave it out. Comparing that word “any” with Romans 8, Les says it includes the whole sphere of creation—not just man, but angelic powers and satanic powers—nothing can pluck them out of His hand. He then applies it: if you can believe God in Christ died and rose again for your salvation, then you should be able to believe these things, that you’re His.