The truth is out their, its everywhere, but everyone is blind, and Satan is always ready to deceive and pull people off of the right path.
Lets take Catholic's for example, i have talked to a couple of Catholics, and they all believe that they don't need to be baptized because they were baptized as a baby. A baby can't repent and a baby can't believe in God. Take symbolism of baptism aside, take the need to get baptized for salvation aside. Take the whole theology from baptism and whether we need it to wash away our sins or not, and put it aside.
What is the core truth?
Why did Jesus get baptized he was sinless?
We do it because it is clear in the bible, that God tells us to repent, and get baptized, its as simple as that, out of obedience to God. Tell them that and they don't know how to respond, because there really is no theological or scriptural debate for it.
Above all though what i have found to be the most effective.
Just invite them out, hey i plan on fasting and praying for a couple days, do you want to join me?
I'm going to do a bible study about a particular subject, lets put theological discussion aside and figure out the truth. Why don't you come around?
The more they see you, the more they hear the word, how you pray, how you read the bible, how close you are to God. How you seem to know answers to things that they don't understand. They start to notice the difference between you and their doctrines, and they start to ask questions.
After all, the light and the truth is in us, we just need to learn to let it shine. That is why its so important, to learn to live in the light, and become an example for people, because that is what people see the most, and that is what is going to draw people to Christ, not scriptural debates.