Philosophical reasoning is an essential tool used by theologians to develop and explain coherent theological concepts, principles, and doctrines. The classical tradition of philosophy remains the primary dialogue partner with theology in the domain of catechetics. And in the encyclical Fides et Ratio by John Paul II, faith and reason are like two wings on which the human spirit soars toward the truth.
In the book When Skeptics Ask by Norman Geisler and Ronald Brooks, they point out the difficulty of speaking from the Word of God as revelation about the Son of God to someone who does not even believe in God. The need for pre-evangelism in a post-Christian world also demonstrates the need for philosophical reasoning as an essential tool used by evangelists to explain coherent theological concepts, principles, and doctrines. The following illustrates how philosophically informed pre-evangelism assists theologically informed evangelism.
Pre-evangelism
1a) Based on reason
2a) Clearing the way for the gospel
3a) To those who don’t believe in God
4a) Preconditions for the gospel
5a) Defends the gospel (Apologetics)
6a) Goal is pre-understanding for the gospel
Evangelism
1b) Based on revelation
2b) Proclaiming the gospel
3b) To those who believe in God
4b) Content is the gospel
5b) Affirms the gospel (Catechesis)
6b) Goal is understanding the gospel
Within divine revelation is the distinction between general revelation and special revelation. The former is revealed to a general audience through nature and is therefore natural. The latter is directly or initially revealed to a limited audience through supernatural means. From a Christian perspective, the former is also known as natural theology and the latter as biblical theology. Natural theology supports pre-evangelism and helps to answer the question: Does God exist? Biblical theology supports evangelism and helps to answer the question: Is Jesus divine?
~ Boethius ~
You may want to read the article “General and Special Revelation” from R. C. Sproul and Keith Mathison. Additionally, the entry “Divine Revelation” found at Stanford Encyclopedia of Philosophy provides detailed information.