To know best how to witness to Catholics, it is good to know some of the things that make Catholics resistant to the idea of being “born again.”
Catholics are indoctrinated from an early age, and a barrier to biblical truth is carefully erected in their minds. Catholics are taught that everything that comes from Rome takes precedence over the Bible. “If the Pope says it, it must be true” is a cultivated mindset. Unfortunately, Catholics are not taught to think for themselves, and many do not know why they believe what they do. Many Catholics have no concept of what is written in the Bible, other than the two or three passages that are read during Mass.
Also, human nature being what it is, any threat to one’s belief system is automatically resisted. Apologetic confrontation tends to make Catholics defensive and to put up walls. To directly attack the apostasy of Catholic teaching is the wrong way; Catholics have been told to expect this from “Protestants,” so most of them are prepared for confrontation or simply cut off communication. Therefore, generally speaking, confronting a Catholic friend with the unbiblical doctrines of his church is self-defeating. It is usually better to gently point him to Scripture and its authority as God’s Word. Never underestimate the power of God’s Word to change a person’s heart (Hebrews 4:12).
The simplicity of the gospel is what will speak to Catholics the most. That’s the “key” in witnessing to them. In many ways, the Catholic Church insulates people from God, who can only be approached through priests and saints, and then only with the proper prayers, penance, and piety. The Bible teaches us “the simplicity and purity of devotion to Christ” (2 Corinthians 11:3, NASB). Jesus extends the invitation to all: “Let the one who wishes take the free gift of the water of life” (Revelation 22:17). “Whoever has the Son has life” (1 John 5:12). Such simplicity appeals to those laboring under a merit-based system of religious works.
Reaching the heart of a Catholic is a gradual process. The armor he wears must be chinked, piece by piece, as doubts arise in his mind about what he has been taught. The idea is to “draw him out” and cause him to ask questions about his own faith. Catholics have to be “spiritually thirsty” in order to search for valid answers. When their questions arise, we want to be in a position to answer them from the Bible. It’s easy to simply condemn what someone believes, but that can easily lead to a lost opportunity for further witness. A Catholic must see the truth for himself.
Of course, it goes without saying that we who witness to Catholics must be in the Word and “prayed up.” We must be compassionate, not antagonistic, and we must let the Holy Spirit guide us. Our prayer should be along these lines: “Lord, You know the heart and the motives of this person. Give me the words she needs to hear.”
As an encouragement, here is a testimony from a former Catholic: “I recall what ministered to me was first hearing the Word several times and then the awesome realization that I could know the Lord personally. For me, all the other Catholic doctrines that were wrong fell away gradually after I was born again and continued to read the Word.”
 Got Questions Ministries. (2010). Got Questions? Bible Questions Answered. Bellingham, WA: Logos Bible Software.