This quote from St. Augustine was posted on another thread but I find that it is quite relevant to this subject:
Saint Augustine, Ex Enarratiónibus sancti Augustíni epíscopi in psalmos (Ps 32, 29: CCL 38, 272-273)
"Those then who tell us: You are not our brothers, are saying that we are pagans. That is why they want to baptise us again, claiming that we do not have what they can give. Hence their error of denying that we are their brothers. Why then did the prophet tell us: Say to them: You are our brothers? It is because we acknowledge in them that which we do not repeat. By not recognising our baptism, they deny that we are their brothers; on the other hand, when we do not repeat their baptism but acknowledge it to be our own, we are saying to them: You are our brothers.
If they say, “Why do you seek us? What do you want of us?” we should reply: You are our brothers. They may say, “Leave us alone. We have nothing to do with you.” But we have everything to do with you, for we are one in our belief in Christ; and so we should be in one body, under one head."
This illustrates perfectly the relationship we have with the Calvinists. We recognize their baptism but they do not recognize ours, yet we still consider them brethren whom we'd like to come into communion with The Church.
Your quote says it all. Moreover, they will have to answer to God for refusing to rightfully
call us Christians (for whatever horribly false reasons they may have); the more that information is available to so many (as in our day), the more culpable will those be who persist in ignorance or blindness. May God show them mercy and grant them the grace to know the truth.
Anyway, yes, they are Christians, even though they adhere to vile
heresy (i.e. predestinarianism, which states that God created certain people specifically and with the intent to send them to hell). But, as Credo said, in general they are material