The misinterpretation of 3:86

How can God guide a people who disbelieved after their belief, when they had already witnessed that the messenger was truthful, and when clear proofs had come to them? God does not guide the transgressing people. 3:86

The Quran contains only one Shahada that is authorised and approved by God:

God bears witness that there is no god except Him, and so do the angels and those who possess knowledge, upholding justice. There is no god except Him, the Dignified, the Wise.
3:18

This Shahada, that has only the name of God, is uttered by God, the angels, and those among the people who possess knowledge.
In contrast, the only ones spoken of in the Quran to utter a Shahada (for prophet Muhammad) are the hypocrites (63:1).

Those who insist that the Shahada for prophet Muhammad is a lawful Shahada, never mention the Shahada in 3:18, nor do they mention the words in 63:1. Instead, they make reference to 3:86 as justification for the Shahada they utter for prophet Muhammad.

They base their claim on the fact that the words in 3:86 contain the words
"witness" and also the word "messenger". Nevertheless, the context in which these 2 words are used in 3:86 is altogether different from what they imply. When we study the words in 3:86 closely we find that they do not refer, directly or indirectly, to the Shahada.

1-
The subject of 3:86 is the disbelievers and their rejection. This fact alone tells us that whatever actions (of the disbelievers) reported in this verse cannot constitute authorisation for any religious act.

It makes no sense to derive religious authorisation from acts of disbelievers! The law of God, the rituals authorised by God, the prohibitions set by God and all other aspects of the religion are given to us in the Quran in the form of direct instructions from God to the believers. These are always in the form of "you shall do" and "do not do". God's law to the believers should never be derived from what the disbelievers say or do.
A comprehensive list of God's law to all believers is found on the following page: A Comprehensive Code of Conduct
The reader will note that none of the items on the list are in reference to disbelievers or what they say or do.

2- Now let us take a closer look at the words "the messenger was truthful". The meaning of these words is clear; they mean that the messenger was a genuine messenger of God, not a fake messenger, and that he came with the truth.
The words in 3:86 say nothing other than assert that God would not guide disbelievers when they denied and rejected even after witnessing that the messenger came with the truth, and after clear proofs had come to them.

The words in 3:86 have nothing whatsoever to do with a Shahada that must be uttered for the messenger.