The answer is that we need to back up our arguments with Scripture. However, when we do, it is often rejected or ignored by the person who asks us questions about God, but at the same time, refuses to acknowledge God's authority contained in His written Word.
In previous posts, I have often discussed the fact that those who are not born again in Christ often think that sharing Bible verses with them is useless, and/or meaningless.
Note this one example that I recently received in the comment section of a previous post:
...Don't pretend to 'teach' them with Spurgeon quotes and marginally apropos Bible exerpts [sic]...
The Bible reveals precisely why such a person would say something like this:
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned. 1 Corinthians 2:14 (KJV)
This is not meant as a put down. I know that those who don't understand the concept of being born again in Jesus Christ might think that this is a "you think you are better than me" kind of verse.
No. That is not the reason. That is not the purpose.
The reason that such a verse (as well as the corresponding verses below) is in the Bible is for instruction. It is a teaching kind of verse that lets believers know what they (we) are up against when attempting to evangelize non-believers with the gospel.
Cross References to this verse:
John 14:17 that is the Spirit of truth, whom the world cannot receive, because it does not see Him or know Him, but you know Him because He abides with you and will be in you.
1 Corinthians 1:18 For the word of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God.
1 Corinthians 15:44 it is sown a natural body, it is raised a spiritual body. If there is a natural body, there is also a spiritual body.
1 Corinthians 15:46 However, the spiritual is not first, but the natural; then the spiritual.
James 3:15 This wisdom is not that which comes down from above, but is earthly, natural, demonic.
Jude 1:19 These are the ones who cause divisions, worldly-minded, devoid of the Spirit. (NASB ©1995)
Those who profess to be Christians, yet refuse to adhere to God's Word, are also leading their lives "in the flesh."
I have seen how confused non-believers are when they can't understand why Christians tell them their need to repent.
It doesn't help, either, that the current post-modern, seeker-sensitive (a.k.a. don't mention repentance of sin and/or the cross of Christ) emergent church, purpose-driven distraction is out there - leading seekers astray and into heresy.
Note what the Treasury of Scripture Knowledge tells us:
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned. (NKJV)
the natural man. [Psuchikos,] the animal man, one who lives in a natural state, and under the influence of his animal passions; for [psuche] means the inferior and sensual part of man, in opposition to the [nous] understanding, or [pneuma,] the spirit.
There are MANY GREAT commentaries written about this verse. Here are two of the best:
Jamieson-Fausset-Brown Bible Commentary
14. natural man-literally, "a man of animal soul." As contrasted with the spiritual man, he is governed by the animal soul, which overbears his spirit, which latter is without the Spirit of God (Jude 19). So the animal (English Version, "natural") body, or body led by the lower animal nature (including both the mere human fallen reason and heart), is contrasted with the Spirit-quickened body (1Co 15:44-46). The carnal man (the man led by bodily appetites, and also by a self-exalting spirit, estranged from the divine life) is closely akin; so too the "earthly." "Devilish," or "demon-like"; "led by an evil spirit," is the awful character of such a one, in its worst type (Jas 3:15).
receiveth not-though they are offered to him, and are "worthy of being received by all men" (1Ti 1:15).
they are foolishness unto him-whereas he seeks "wisdom" (1Co 1:22).
neither can he-Not only does he not, but he cannot know them, and therefore has no wish to "receive" them (Ro 8:7).
The previous commentary informs us why it is necessary for people to have faith and believe in Jesus, realize the truth in what Christ did for us at the cross, confess sin and repent FIRST, and then receive Christ as Lord and Savior. In that order, salvation works. Out of that order, it doesn't.
Matthew Henry's Concise Commentary
2:10-16 God has revealed true wisdom to us by his Spirit. Here is a proof of the Divine authority of the Holy Scriptures, 2Pe 1:21.
In proof of the Divinity of the Holy Ghost, observe, that he knows all things, and he searches all things, even the deep things of God. No one can know the things of God, but his Holy Spirit, who is one with the Father and the Son, and who makes known Divine mysteries to his church. This is most clear testimony, both to the real Godhead and the distinct person of the Holy Spirit.
The apostles were not guided by worldly principles. They had the revelation of these things from the Spirit of God, and the saving impression of them from the same Spirit. These things they declared in plain, simple language, taught by the Holy Spirit, totally different from the affected oratory or enticing words of man's wisdom.
The natural man, the wise man of the world, receives not the things of the Spirit of God. The pride of carnal reasoning is really as much opposed to spirituality, as the basest sensuality.
The sanctified mind discerns the real beauties of holiness, but the power of discerning and judging about common and natural things is not lost.
But the carnal man is a stranger to the principles, and pleasures, and actings of the Divine life. The spiritual man only, is the person to whom God gives the knowledge of his will.
How little have any known of the mind of God by natural power!
And the apostles were enabled by his Spirit to make known his mind. In the Holy Scriptures, the mind of Christ, and the mind of God in Christ, are fully made known to us.
It is the great privilege of Christians, that they have the mind of Christ revealed to them by his Spirit. They experience his sanctifying power in their hearts, and bring forth good fruits in their lives.