Apostle Paul or Paul the apostle?

Why do all the Sabbath-keeping Church of God groups refer to the "Apostle Paul" with a capital letter rather than the apostle Paul or Paul the apostle? I'm not trying to be picky but biblical! What's the difference between the religious title of APOSTLE versus RABBI? We know Yeshua said not to refer to any man as a rabbi, using it as a religious title, didn't he? Isn't Yeshua our Apostle and the others are apostles? Ever think about it? Doesn't the same principle apply when it comes to any religious title?

Hebrew 3:1
Therefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our confession, Christ Jesus...

Matthew 23:6-8
6 They love the best places at feasts, the best seats in the synagogues, 7 greetings in the marketplaces, and to be called by men, ‘Rabbi, Rabbi.’ 8 But you, do not be called ‘Rabbi’; for One is your Teacher, the Christ, and you are all brethren.

1 Corinthians 4:7
For who makes you differ from another? And what do you have that you did not receive? Now if you did indeed receive it, why do you boast as if you had not received it?

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