What is the biblical basis for “the Sinner’s Prayer” and do Protestants believe that saying the prayer make one a Christian?

I agree with you brother that they need to have fruit of Holy Spirit as sign to be Christians.

So for a whole year Barnabas and Saul met with the church and taught great numbers of people. The disciples were called Christians first at Antioch [Acts 11:26].

Barnabas and Saul teach and work them with Antiochians for the whole year. And then they yield fruit and people saw Christ in them and then called them Little Christs.

Maybe, Sinner Prayer is popular with ministers because they could brag about bunch of unsalty salt, which Jesus like less than manure [Lk 14:34-35], instead of working hard like Saul. Maybe it would be something, if you would pray Sinner Prayer and act as a publican every day.

On other side, in my Catholic church, from the early days (4 century and before), if you want to be just baptized, and not like going to die soon, you need to be disciple for one whole year to learn what you step in, just like above. There had porters, church bouncers, which expel those people on second. And from my personal experience and many testimonies grace by blood of Jesus is much better communicated when you need to confess your sin to other person [James 5:16] and also have prescribed penance [John 21:15-19]. It is so much powerful that even evangelical preachers (Francis Chan, just to mention someone) reinvent the wheel. Don't want to insult or hurt someone, but isn't such bad evangelization as Sinner Prayer natural product after Martin Luther's salvation by faith alone (for which I argue that is too not biblical [James 2:24]) and once saved always saved and Bible interpretation relativism through Sola Scriptura and 30000+ denomination?

The problem with presenting a good answer to your question is that word "protestant".

A member or follower of any of the Western Christian churches that are separate from the Roman Catholic Church and follow the principles of the Reformation, including the Baptist, Presbyterian, and Lutheran churches.
-- Oxford Dictionary (from Google)

That's a very broad range of philosophies! As you can see just from the definition, many of these groups hold contradictory beliefs. Nobody can answer your question for all of them at once.

In fact, some do believe that all you need to do is to say a prayer. Others consider that philosophy fatally flawed. Most seem to fall somewhere in between. With all these different philosophies and backgrounds, you're going to get all sorts of different explanations.

Are we all wrong? Is anyone right? You asked the right question. "What is the Biblical basis ..."

My advice is to trust nobody (not even me) and stick to the original source until you understand from that who is telling the truth. Quoting Jesus, "The path is wide that leads to destruction." So finding the right person (or group) who WON'T feed you lies is like finding hay in a stack of poison needles.

1 Cor. 15:1-4 Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, 2 by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain. 3 For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He was raised on the third day according to the Scriptures

Salvation has always been by grace (God's unmerited favor) through faith. See Romans 3-4, entire gospel of John. In 1 Cor. 15 we have the clearest presentation of the gospel post resurrection. The key word for salvation is always 'believe'.

The sinner's prayer is basically acknowledging the first requirement of the gospel in that we are sinners and need a savior. Saying thank you to Jesus is a simple way to acknowledge that one is receiving the gift of eternal life through faith. Saying thank you to Jesus also acknowledges that he's alive or resurrected.

A person receives the Holy Spirit the moment they believe: John 7:39, Acts 19:2, Eph 1:13-14

I grew up in a church that believed in the Sinner's Prayer. The verse most often associated with it was Romans 10:9 (emphasis mine):

9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.

This Scripture implies that there is a speaking component to being saved. I believe some Christians have used this to base the Sinner's Prayer practice, as it fulfills a speaking component to being saved.

What has become known as “The Sinner’s Prayer” in Protestant circles is a comparatively new phenomenon. In some groups non-members who listen to preaching are urged to "say the sinner's prayer" and accept Jesus as Lord and Saviour - then that person is told they are now saved from their sins. Many have thought that because they repeated 'the sinner's prayer' after hearing a gospel message, they can then sign a bit of paper saying they've done that, adding the date when "they decided to accept Christ", and so they assume they are now Christians. If a distinction is made between salvation and discipleship, then a dreadful error grows, for the idea of salvation without commitment to living a sanctified life has led to easy-believeism, and 'cheap grace'. The new recruit is encouraged to rejoice and to relax, and to now ‘live for Christ’ by doing what the members do and believing what the members teach them. Yet, because they are now ‘saved’, that becomes some kind of optional extra, in their thinking.

It appears that this developed from the early 20th century, influenced by Lewis Sperry Chafer's teachings which introduced the notion that it is possible to be saved yet see nothing of the outworking of regeneration in one’s life-style or behaviour. In 1918 he published ‘He That Is Spiritual’, claiming that the Bible speaks of two classes of Christians: carnal and spiritual, the former having ‘a walk’ that is on the same plane as that of the ‘natural’ [unsaved] man. He was a leader in the 'dispensational theology' school. Dispensationalism is a fundamentally correct system of understanding God's dealings with humanity through the ages. However, a problem is that enthusiasts can compartmentalise the truth to the point of making unbiblical distinctions. Obsession with categorising everything causes hard lines where the Bible has none. Chafer promoted the view that,

"to impose a need to surrender the life to God as an added condition of salvation is most unreasonable." (Systematic Theology 3:385 Dallas, Dallas Seminary 1948 & writings in 1918 & 1967)

Here his mistake was making a distinction between salvation and discipleship. Chafer seemed to think that surrendering one's life to Christ as Lord and Saviour is added (by others) as a ‘process’ of salvation. It is not. But this idea of salvation without commitment to living a sanctified life has led to easy-believeism, and 'cheap grace'. Dr. Chafer’s teachings have become the basis for a whole new way of seeing the gospel.

The dispensationalist division between the age of law and of grace has caused confusion about the doctrine of salvation. Another dispensationalist writer, Clarence Larkin, wrote in his book ‘Grace’ (p 132, Grand Rapids: Zondervan 1922) that the gospel Jesus preached had nothing to do with salvation but was simply an announcement that the time had come to set up the kingdom of Christ on earth. He also claimed that the teachings of the Sermon on the Mount

"have no application to the Christian, but only to those who are under the Law, and therefore must apply to another Dispensation than this." ('Dispensational Truth' p87, Philadelphia, 1918)

His books and charts are still used today by many Dispensationalists. This kind of teaching has emasculated the gospel due to the idea that people just hear a gospel presentation, say the sinner's prayer, and then they are saved. Evangelistic ‘appeals’ that use phrases like, ‘make a decision for Christ’, ‘ask Jesus into your heart’, ‘accept Jesus as your personal saviour’ violate the spirit and the terminology of the biblical summons to unbelievers. The Bible commands sinners to repent and follow Jesus. Verbal permission for Jesus to ‘enter in’ misconstrues Jesus’ words in Revelation 3:20 (‘Behold I stand at the door and knock…’) because first comes his command in verse 19, ‘Be zealous, therefore, and repent.’ Only then will the truly repentant find Jesus coming in to them. This and the previous paragraph gleaned from John F MacArthur Jr’s. book ‘The Gospel According to Jesus’ pp17-30, 89, 106 (Word of Grace, 1988).

My answer has given examples of grave misunderstandings about becoming a Christian, which arose at the turn of the twentieth century, giving rise to the now-popular “Sinner’s Prayer” notion. The negative aspect of this is what does NOT make a person a Christian, namely, repeating “the Sinner’s Prayer”. The Holy Spirit must convict a person of their sin before they can truly repent but some Protestants seem to think they can rush that divine process by a formulaic prayer-statement after the most cursory (and therefore cursed) gospel message (Galatians 1:6-9). Being born again necessitates a preparation by God Almighty himself, by the words of the preparative messenger which he himself authorises and sends. Receiving these words, there will be a cleansing that will separate a soul and remove from the soul that which prohibits the growth of the word of God within it. This is the beginning of the gospel. The gospel of God speaks to the changed mind, for the carnal mind is at enmity with God. Repeating “The Sinner’s Prayer” does nothing to replace a carnal mind with “the mind of Christ” (1 Corinthians 2:16).