In Ephesians 1:13, believers were sealed with what?

Answer: The promised Holy Spirit

Having heard and believed the gospel, 'you were sealed with the promised Holy Spirit' (KJV: 'that holy Spirit of promise').

“And in Him, having heard and believed the word of truth—the gospel of your salvation—you were sealed with the promised Holy Spirit,”— Ephesians 1:13 (BSB)