As I have explained many many many many many many times I am an ex Catholic. Unfortunatley, in some ways my Family is not the run of the mill Catholic family. They actually know Catholic Doctrine and I have been arguing with them ever since I left the Catholic Church 25 years ago. Because of it. I actually know what the catholic church actually believes rather than the condenced format most protestants hold to from years of misinformation, to outright lies from sensationalist. I've used no mumbo jumbo but quoted their source text directly. Because I have the intelligence to 1) accurately describe what it is they believe 2) I can argue it with out throwing in nonsence view which can and are easily dismissed because they "don't believe that anyway". Often Catholics have the same view that Christians do when muslims argue against them.
Truthfully, I don't have to know a thing about another's religion. What I do have to know is the Bible. No matter what religious person I witness to I must have a good working knowledge of the Bible.
1 Peter 3:15 But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear:
I can give an answer (defense) without studying their beliefs. Many missionaries have been put in that position. The gospel is not one of retreat and defend. It is a message of go forth and preach.
A J.W. came to my door the other day. Though I know quite a bit of their beliefs I stayed away from them. I simply asked them "Have you been born again." They said "no." So I kept right on that topic. Unless you are born again you cannot enter the kingdom of God. It doesn't matter at that point what their view of Christ is. They must be born again. They must see that necessity. Never mind about eschatology, the 144,000, etc. They must be born again. Don't get side-tracked.
For instance a muslim will say you believe such and such when in fact you don't. How do you respond? Well you say "we don't believe that" And when they are able to accurately describe what it is you believe then you can have a real debate. Same thing here.
So what if they don't believe that. What do they believe about eternity? Some day they will die. Then what? What about forgiveness of sins? How do they know Allah will forgive their sins? Do they have any guarantee? Get them thinking. One must be that way with any individual that you witness to.
One Muslim told me: "You believe in the trinity don't you?" Then I replied, "Yes, but what do you think I mean when I say 'I believe in the trinity'"? He replied--3 Gods. He gave the standard answer his Muslim cleric had drilled into him what Christians believe, which is false of course. I took two hours to clear up those misconceptions. We, like them, believe in only one God.
I don't have to know their religion. I have to know my Bible. Knowing their religion gives some advantage, but not a great deal, not as much as a good knowledge of your own Bible.
Hebrews 4:12 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.