“True believers will stand on the words of God alone.”
Your variety of true believer stands on a selected few of the words of God, and rejects many gifts that God answers.
Some people, it would seem, are not capable of reviewing evidence against their positions with an honest, enquiring mind.
2 John 1:9 Anyone who goes too far and does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he has both the Father and the Son. 10 If anyone comes to you and does not bring this teaching, do not receive him into your house, and do not give him a greeting; 11 for the one who gives him a greeting participates in his evil deeds
Galatians 1:8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
Please show where Christ or the apostles taught the assumption of Mary.