Your argument in a nutshell,
Not tithing is considered a sin.
Here is an experiment:Go to a local church and when the collection plates are passed around to gather the offerings and tithes, examine with your very own eyes how many actually give tithes and how many fail to do so.
Since a "Pastor" is considered the shepherd of his congregation, he has the responsiblity to instruct his flock. Therefore he has the duty to tell his congregation and to all those who fail to give tithes that they "ARE IN FACT SINNING"
I actually encourage those who are "able" to give to give out of the abundance of their heart, that is IF they have the means, "Give what you have, not what you do not have" However, because they fail to do so doesn't give the right to throw guilt trips upon them.
But Peter said, Ananias, why hath Satan filled thine heart to lie to the Holy Ghost, and to keep back part of the price of the land? Whiles it remained, was it not thine own? and after it was sold, was it not in thine own power? why hast thou conceived this thing in thine heart? thou hast not lied unto men, but unto God. Acts 5:3,4
It is up to the persons heart to decide whether they want to give out of love and gratitude, not because it is a
sin not to do so. Ananias did not have to give to the congregation, because the land was his which God have have him, his blatant lie against the Holy Ghost is what caused his demise.
God Bless!