When sharing the gospel clearly we should have two great concerns. First, we should be absolutely clear about the condition of salvation - believe. That belief must be free from any works, commitment, or idea of merit on our part so that grace remains grace.
The second concern is that we should be clear about the content of the gospel, or what has to be believed. The content of the gospel is the person and work of Jesus Christ, which are inseparable as the object of saving faith.
We are saved by Someone, the Lord Jesus Christ. Not just any Jesus, but the One sent from God who is the Son of God. There are many things implied by the designation Lord Jesus Christ such as deity, humanity, and messianic mission. While someone may not comprehend a full-blown Christology, there must be some understanding of Jesus' uniqueness and divine authority. The Gospel of John, recognized for its evangelistic intent (John 20:30-31), emphasizes the deity of Jesus more than any other Bible book (e.g., John 1:1-3, 14, 18; 5:17-21; 6:69; 7:38; 8:19, 58; 10:30; 20:28). In John the person of Jesus Christ is the object of faith in many evangelistic contexts (e.g., John 1:12; 3:16; 5:24; 6:29, 47; 9:35-37; 11:25-26).
As the Son of God, Jesus saves us by what He did for us; He provided for our greatest need. We are after all, saved from something and to something. As sinners separated from God, we needed someone to pay the penalty that we could not pay. Jesus paid that price by dying on the cross. Of course, a dead savior could save no one, so Jesus rose from the dead. His resurrection shows that the price has been paid, that God accepted the payment, and that He lives to give us eternal life. Jesus made it possible for us to pass from death into life if we accept His provision (John 5:24).
The person of Jesus can not be separated from His work. Jesus is the "Lamb of God" who takes away the sin of the world (John 1:29). He gave His life for us (e.g., John 6:51; 10:11-18) as the supreme sacrifice for our sins (Heb. 10:5-10). Isaiah 53 speaks prophetically of Jesus' substitutionary death (Isa. 53:3-12) and His resurrection (Isa. 53:10-12).
Again, there are many deep and profound intricacies surrounding Jesus' death and resurrection which an unsaved person may not grasp immediately. Yet it seems there must be at least the simplest understanding that we are sinners separated from God, that Jesus removed the barrier caused by that sin through His death and resurrection, and that He now lives to give us His life. That is why we see the preaching of the cross and the resurrection in the early church (e.g., Acts 2:23-24, 36; 3:18-20; 4:2, 10; 5:29-31; 10:39-40; 13:29-30; 17:3; 26:22-23) and why those great facts were reiterated in the epistles (e.g., Romans 3 - 8, 1 Cor. 1:18-24; 2:1-2; 15:1-4; Gal. 3:1; Eph. 1:20; Phil. 2:8-9; Col. 2:12- 14; Hebrews; 1 Pet. 1:3, 18-21; 3:18).
It is certainly conceivable that a person can understand the facts about the person and work of Christ and yet not be saved because he does not apply them to his own spiritual condition. We believe Christ for something, and that is eternal life. God has promised us that whoever believes in Jesus Christ as the One who died and rose again will have eternal life (e.g., John 1:12; 3:16; 5:24; 6:40, 47; 7:38; 10:26-29; 11:25-26; 12:44-50; 20:31). A person must believe, or be persuaded, that the promise is true and true for himself.
Eternal life has many implications that a person may not fully comprehend. It encompasses eternal security, forgiveness of sin, justification, new birth, glorification, and other wonderful truths that will become clearer with instruction in God's Word. Eternal life is also defined as knowing God through Christ (John 17:3). A person must believe God's promise for some salvific aspect of this eternal life.
While the gospel's content is essentially simple and we can share it clearly, questions may remain about certain situations: How does a very young child understand the gospel's content? How can a mentally impaired person be saved? What happens to babies who die without any knowledge of the gospel? How does a Hindu understand the concepts of God, sin, Son of God, resurrection, eternal life? When we share the gospel, we must realize that the communication process has two components, the communicator and the one who receives it. Not always does the listener process the information exactly as the communicator means it. In other words, there are barriers in communicating the gospel such as language, cultural interpretation, attentiveness, clarity, processing, pre-understanding, and religious preconceptions.
In light of these unknowns, we must humbly acknowledge that a person's understanding may not always be what we think it is. Thankfully, the Holy Spirit knows what we do not. While it is our responsibility to share the gospel as clearly as possible, it is His work to convince (convict; John 16:8) the hearer of its truthfulness. As we share the gospel, we must depend on the Holy Spirit to work in the hearer to give enough understanding to bring about faith (Rom. 10:14-17). Exactly how the Spirit works in a person's understanding will always remain somewhat a mystery (John 3:8; 6:44-45, 65). None of this, however, takes away from the fact that if we are wrong in our message, the listener will be wrong in his faith.
We are called to share the gospel of salvation which means that we share the person, provision, and promise of Jesus Christ. Why would we share anything less? Whether we explain it in the most basic terms or in great depth, it is always the Holy Spirit who brings the understanding which prompts faith. We preach the gospel of grace through faith alone in Christ alone and allow God to do the rest. Certainly that requires us to communicate it clearly, but also to pray fervently.