Jesus certainly didn’t think that becoming a disciple was a secondary, optional step for believers. His three requirements for discipleship that we read in Luke 14 were not addressed to believers as an invitation to a higher level of commitment. Rather, His words were addressed to everyone among the multitudes. Discipleship is the first step in a relationship with God. Moreover, we read in John 8:
As He [Jesus] spoke these things, many came to believe in Him. Jesus therefore was saying to those Jews who had believed Him, “If you abide in My word, then you are truly disciples of Mine; and you shall know the truth, and the truth shall make your free” (John 8:31-32).
No one can intelligently argue against the undeniable fact that Jesus was talking to newly-professing believers about being His disciples. Jesus did not say to those newly-professing believers, “Sometime in the future you may want to consider taking the next step, a step of commitment, to become My disciples.” No, Jesus spoke to those new believers as if He expected them to be disciples already, as if the words believer and disciple were synonymous terms. He told those newly-professing believers that the way they could prove they were His disciples was by abiding in His word, which would result in their being set free from sin (see 8:34-36).
Jesus knew that just people’s profession of faith was no guarantee that they really did believe. He also knew that those who truly believed He was the Son of God would act like it—they would immediately become His disciples—yearning to obey and please Him. Such believers/disciples would naturally abide in His Word, making it their home. And as they discovered His will by learning His commandments, they would be progressively set free from sin.
That is why Jesus immediately challenged those new believers to test themselves. His statement, “If you are truly My disciples” indicates He believed there was a possibility that they were not true disciples, but only professing disciples. They could be fooling themselves. Only if they passed Jesus’ test could they be certain they were His true disciples. (And it seems from reading the rest of the dialogue in John 8:37-59 that Jesus certainly had good reason to doubt their sincerity.)
Our key scripture, Matthew 28:18-20, itself dispels the theory that disciples are a higher class of committed believers. Jesus commanded in His Great Commission that disciples be baptized. Of course, the record of the book of Acts indicates that the apostles didn’t wait until new believers took a “second step of radical commitment to Christ” before they baptized them. Rather, the apostles baptized all new believers almost immediately after their conversion. They believed that all true believers were disciples.
In this regard, those who believe that disciples are the uniquely committed believers are not consistent with their own theology. Most of them baptize anyone who professes to believe in Jesus, not waiting for them to reach the committed level of “discipleship.” Yet if they really believe what they preach, they should only baptize those who reach the discipleship level, which would be very few among their ranks.
Perhaps one final blow to this diabolical doctrine will suffice. If disciples are different than believers, why is it that John wrote that love for the brethren is the identifying mark of true born-again believers (see 1 John 3:14), and Jesus said that love for the brethren is the identifying mark of His true disciples (John 13:35)?
 This passage of Scripture also exposes the mistaken modern practice of assuring new converts of their salvation. Jesus did not assure these newly-professing converts that they were surely saved because they had prayed a short prayer to accept Him or verbalized faith in Him. Rather, He challenged them to consider if their profession was genuine. We should follow His example.