There is no nothing (That I came across of know of) in the Quran and Sunnah any thing that limits Kind treatment and charity to Muslims only, All verses that speak of charity and good act do not only limit these interactions to a single specific group. Some of these verses are quoted below:
Allaah does not forbid you to deal justly and kindly with those who
fought not against you on account of religion nor drove you out of
your homes. Verily, Allaah loves those who deal with equity”
[al-Mumtahanah 60:8]
In Surat Al Insan, as shown in the verses quoted below, yet again God does not specify who charity is given to:
They [are those who] fulfill [their] vows and fear a Day whose evil
will be widespread. 76:7 And they give food in spite of love for it to
the needy, the orphan, and the captive 76:8 [Saying], "We feed you
only for the countenance of Allah . We wish not from you reward or
gratitude. 76:9
Yet again in Surat An-Nahl:
Surely Allah enjoins the doing of justice and the doing of good (to
others) and the giving to the kindred, and He forbids indecency and
evil and rebellion; He admonishes you that you may be mindful(16:90)
And also in Surat Al Baqara:
Righteousness is not that you turn your faces toward the east or the
west, but [true] righteousness is [in] one who believes in Allah , the
Last Day, the angels, the Book, and the prophets and gives wealth, in
spite of love for it, to relatives, orphans, the needy, the traveler,
those who ask [for help], and for freeing slaves; [and who]
establishes prayer and gives zakah; [those who] fulfill their promise
when they promise; and [those who] are patient in poverty and hardship
and during battle. Those are the ones who have been true, and it is
those who are the righteous. 2:177
Unfortunately many Muslims have become closed minded, as they have created a division between them and other nations this could be due to many causes such as nationalism or viewing their culture OR tribe as superior, this has an influence on the way they think and put forth opinions, however this is against what God instructs us in the Quran:
O mankind! We created you from a single (pair) of a male and a female,
and made you”INTO NATIONS AND TRIBES” that ye may know each other (not
that ye may despise (each other). Verily the most honoured of you in
the sight of Allah is (he who is) the ”MOST RIGHTEOUS OF YOU” . And
Allah has full knowledge and is well acquainted (with all things).
[49:13]
Our religion does not demand its followers to be blind, be careful of where you receive knowledge and answers from, always verify what you have been taught with its source:
"And follow not that whereof you have no knowledge. Surely, the
hearing, the sight, the heart--all of those shall be questioned
of."Al-Quran 17:36
If you do not have complete knowledge about anything, better keep your
mouth shut. You might think that speaking about something without full
knowledge is a trivial matter. But it might have grave consequences
[24/15-16]
Coming to the last part of your question,if you know that your charity , you shouldn't give blindly, investigate to see what cause you are aiding then give if it is a righteous cause:
Help you one another in Al-Birr and At-Taqwa (virtue, righteousness
and piety); but do not help one another in sin and transgression
[al-Maa’idah 5:2]
Although the charity was christian, its cause was still a righteous cause, so as long as you know that you donations are going to fulfil that cause then there should be no issue, but if it is to fund-raise missionary campaigns and things of that nature then this is an issue.