Preaching the Gospel, Preparing a People
I am sorry to tell you about this but the article is not on easter a practice based in paganism it is instead about God’s holy days as outlined in Leviticus 23.
We use the old testament sources to back up Jesus Christ but without Jesus to fulfill those prophecies, they are useless in proving anything without new testament fulfillment. This is because of the nature of the predictions, in this case pointing to Jesus. To answer your second question the true religion switching from Judaism to Christianity does not invalidate the promises to Abraham. I hope I have explained this well enough for you to understand.