The 'signs' was given in Mark 16
6He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
17And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
18They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.
No flames given, or the sound as of a mighty rushing wind either .
Why do you not have the sound from heaven as of a mighty rushing wind recreated today in church ? Why do folks expect the ' tongues ' and not the sounds and appeared unto them cloven tongues like as of fire, and it sat upon each of them ? Why is everyone cherry picking the speaking in ' tongues ' part but don't expect the sounds and appearances of cloven tongues AS OF fire ,today ?
Not wanting to turn this into a discussion on the gift of tongues but do you really not know the answer to that question? The reason that pentecostals say that speaking in tongues is the initial physical evidence of being baptized in the Holy Spirit is that it was the common sign that happened more than once when believers were baptized in the Holy Spirit. The tongues of flame and rushing mighty wind happened only once. The gift of tongues happened on each occasion in the rest of the book of Acts.
This doctrine "initial evidence" started when the question was raised by Charles Parham to a group of bible college students to study the book of Acts and to determine what was the evidence of the Baptism of the Holy Spirit based on these events in the book of Acts. They all agreed that it was the gift of tongues. This doctrine was then further developed through the years and became the "initial Physical evidence" As the answer to the question what was visible in these events. The flames only happened once and was not present at the other events. Therefore it would not do to say that tongues of flame must set upon each because that did not happen in each case in the book of Acts.
There were things that happened, things that were seen and heard at each event we call baptisms in the Holy Spirit in the book of Acts. List them:
Tongues of flame,
Rushing mighty wind,
Declaring the wonderful works of God
Prophesying
Speaking in tongues
But what happened in all of them? Speaking in tongues.
That is why it is taught that speaking in tongues is the initial physical evidence. Not the only evidence. There are other gifts of the Spirit that come as a result. But they are not all manifest initially whereas tongues did seem to be the common sign.
Now you may not agree with it, and that is your decision but that is the reason why they say that speaking in tongues is the initial physical evidence for each person who is baptized in the Holy Spirit. They did not mean that it was the first sign mentioned in the Bible. It was the answer to the question "How can we know that someone is receiving the same thing they did in the book of Acts" The answer is "They spoke in tongues in each event recorded in Acts that we commonly call a baptism in the Holy Ghost event"
Now it is true that a couple of these events did not mention tongues such as the Samaritans but the fact that Simon saw something that made him want to offer money to have the gift to lay hands on people to receive the gift proves that there was a physical manifestation that impressed him. No one would offer money for a gift that was not obvious to anyone.
We know that Paul spoke in tongues more than all the Corinthians and when did he receive it? When Anaias laid hands on him to receive the Holy Ghost.
Nevertheless we have Pentecost, the Ephesus believers and the house of Cornelius that all record all of them speaking in tongues when they received the baptism of the Holy Ghost. This is enough for most people to conclude that the speaking in tongues should be expected and is the initial physical, outward evidence that one is being baptized in the Holy Ghost.
Now I know that you and others here don't agree and I don't want to turn it into a thread about it. I was answering your question as to WHY they call it initial evidence instead of flames and wind. You seem to think that by "initial" they meant the first time it appears in the bible. That is not what they mean. There are many results of being baptized in the Holy Ghost. Other gifts of the Spirit for example such as prophesy. Is it biblical to say when someone in the book of Acts was baptized in the Holy Ghost they always prophesied? Not unless you equate speaking in tongues to prophesy. Is it biblical to say that when anyone in the book of Acts was baptized in the Holy Ghost they spoke in tongues? Many believe that is true and that is the meaning of the "initial physical evidence of being baptized in the Holy Ghost is speaking in Tongues. Acts 2:4, 1 Cor 12:4-10,28