James 2: 22 You see that his faith and his actions were working together, and his faith was made complete by what he did. 23 And the scripture was fulfilled that says, “Abraham believed God, and it was credited to him as righteousness,” and he was called God’s friend. 24 You see that a person is considered righteous by what they do and not by faith alone.
Genesis 12: 1 The LORD had said to Abram, “Go from your country, your people and your father’s household to the land I will show you.
2 “I will make you into a great nation,
and I will bless you;
I will make your name great,
and you will be a blessing.
3 I will bless those who bless you,
and whoever curses you I will curse;
and all peoples on earth
will be blessed through you.”
Witnesses take the Bible's "plotline", so to speak, very seriously and of course as literal history. The above scriptures indicate that, according to the Bible, God chooses people to become friends with even so far as to curse their enemies. The Watchtower is drunk with the idea that their entire organization, including all doctrines and policies, are enough to make them "friend's of God."
So asking them whether or not someone can become a Witness only using the Bible will be an easy question for them to answer as they firmly believe that they are God's chosen people on earth just as Abram and his decendents were chosen in the ancient world.
Cult personalities are designed to be hard nuts to crack.