All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Calling both the Muslims and the non-Muslims is obligatory and it is an obligation to be concerned with all of them and not ignore either party at the expense of the other. One must call the non-Muslims to Islam so that they would embrace it and come out of the darkness of disbelief to the light of guidance and faith, and one must also call the believers so that they would do what Allaah has ordained them to do.
Indeed, Allaah informed us that He sent Moosa (Moses) and ‘Eesa (Jesus), may Allaah exalt their mention, to both parties: to those who were originally believers but deviated (from the straight path) in their life, and to the non-Muslims, like Pharaoh and the like. Allaah Says about Moosa in regard to Pharaoh (what means): {“Go to the wrongdoing people. The people of Pharaoh. Will they not fear Allaah?”}[Quran 26:10-11] Also, Allaah told him in regard to the children of Israel (what means): {And We certainly sent Moosa (Moses) with Our signs, [saying] “Bring out your people from darkness into the light and remind them of the days of Allaah.”}[Quran 14:5]
In addition to this, the Prophet used to spend most of his time in teaching, making Da’wah and educating the believers and purifying them, but he was also very concerned about the non-Muslims; he used to visit them in their places of gathering and call them to Islam and make Jihaad with them with the word and the sword until they embraced the religion of Allaah in multitudes.
As regards Chapter ‘Abasa [Quran 80] it is not a textual evidence about giving priority to the believers at all times, but it is evidence about not neglecting someone who wants guidance because of being preoccupied by calling the heedless people who deny the truth.
Therefore, the Muslims should endeavor to call both parties and the people in charge of making Da’wah should look to what is suitable for each situation so that the obligation should be fulfilled in regard to both parties. So, the Muslim who came willing to ask about the rulings of his religion and the Muslim who is feared to go out of the fold of Islam come in priority over the non-Muslim who is still denying the truth and neglecting it, as it is the case of the non-Muslims who were mentioned in the question. On the other hand, the non-Muslim who is hoped to embrace Islam comes in priority over a Muslim who is holding tight to his religion and who wants to gain more knowledge about his religion. Therefore, it is the circumstance that determines who comes in priority.
Allaah Knows best.