I'm sorry but if someone knows the basics even of Christianity in general I would think they know there is only one God.
You'd think that, but then there are other solid core principles of Christianity that I've seen a number of "Christians" get wrong such as, say, repentance? How many people again, believe in "Free Grace", where they can say "I'm sorry" on Sunday and then go back to doing whatever it was they were doing wrong in the first place all over again and repeat this process every single week of their "Christian" lives? You'd think anybody who knows the basics of Christianity would know better, but... apparently not. You can't assume people know stuff.
Christians are to lead by example. We are called to live holy and separated lives. The world's ways are not God's ways. We are called to follow His ways, and while none of us are perfect, we can at least try our best to lead a holy, Christian life. Our life is the biggest witness to our faith that we have. A person who professes Christ, but goes out and do things no Christian should do (getting drunk, getting rowdy at sports games, swearing, etc) is only sullying Christ's name.
Gandhi once said "I like your Christ, but I do not like your Christians for they are nothing like your Christ."
Let that sink in for a moment. Gandhi refused to become Christian,
because of people who SAY about Christ, but don't DO what Christ did. They are hearers, and sayers of the word... but they aren't
doers.
When you go to another religion's Temple, for entertainment purposes or curiosity... what does that say about you and your faith? Seriously, you're
curious about a false religion? If you're a Christian, and you claim you have faith in Him, then you
know you've found your Path. Why are you
curious about any other path, which you (should) know is wrong?
EDIT: The 'yous' in this paragraph are hypothetical and not aimed at anyone in specific.
What does that say to other people who witness "Christians" visiting and watching, and learning about other gods? It shows a lack of faith in your own God. Do you really want to be telling other people that you're full of nothing but vain words when you try to witness to them about the One True God, and then you go out and go exploring amongst other false gods?
Is that the message you want to send? If you want to go to other Temples to witness the Faith, then
one should stand outside of the Temple and offer their words to people coming and going. There's no reason to go
inside the Temple.