What should we tell someone who has just made a decision for Christ. Should we assure them of their salvation? Should we tell them that they should never doubt their salvation because God’s word is true? It seems to me that if God’s word is true, we should warn them that fruit will follow. (Matthew 3:8) Christians are not perfect, but they are certainly changed. Having made a “decision” for Christ, or having “accepted” the Lord really means nothing if our hearts have not been changed. If a person doesn’t find in their heart a love for God and his ways, and an abhorrance of sin; if they can go on living just as they once did without any qualms, I think the Scriptures testify that there is every reason to doubt their conversion. Assurance is something that the Holy Spirit gives. We cannot see into the heart.
The quote below is from Paul Washer.
“If a person makes a profession of faith in Christ, it is appropriate to rejoice with him, but not without further admonition and warning. He must be encouraged by the truth that if he has genuinely repented and believed he is saved. However, he must also be instructed that if his conversion is genuine, it will be further validated by his continuation in the things of God. He must know that salvation is not a one-time vaccination that no longer requires attention. If a person has truly repented unto salvation, he will continue repenting and growing in an ever-deepening repentance throughout his life. If he has truly believed unto salvation, he will continue believing and growing in dependence upon God. In other words, the evidence that God has truly begun a work in a person’s life through conversion is that He continues working in him throughout the entirety of his life. If after a supposed conversion experience a person returns to apathy in the things of God, makes no progress in godliness, and abandons the community of faith, he has no biblical grounds for confidence that his conversion experience was genuine or that he was ever a child of God. It is a great and dangerous error to send new converts away without such a warning.”
–Paul Washer, Gospel Assurance & Warnings, p. 164