peace,
I think 'muhlisan lahuddeen' is a rarely discussed Qur'anic term
[39:2] We have sent down to you the Book with the truth, so serve God faithful/loyal/devoted the system to Him.
[39:11] Say: "I have been commanded to serve God, faithful/loyal/devoted the system to Him."
[39:14] Say: "God is the One I serve, faithful/loyal/devoted my system to Him."
And they were not commanded except to serve God and be faithful/loyal/devoted to His system, monotheists, and hold the swalat and contribute towards purification. Such is the straight/ valuable system. [98:5]
Say: "My Lord orders justice, and that you set your faces at every temple/Masjid, and that you call on Him, while being faithful/loyal/devoted to Him in the system; as He initiated you, you return." A group He has guided and a group have deserved misguidance; that they have taken the devils as allies besides God; and they think they are guided! [ 7:29-30]
[15:39-42]
He(Iblees) said: "My Lord, for that by which You have caused me to be misled, I will beautify for them (what is) on the earth, and I will mislead them all."
"Except Your servants from among them, the faithful/loyal/devoted ones."
He(Allah) said: "This shall be a straight path to Me."
"For My servants, you shall have no authority over them, except those who are misled and follow you."
Also see (40:14,65, 12:24, 19:51, 37:40,74,128,160,169)
So what is 'muhlisan lahuddeen'? Qur'an demonstrates it in the following verses
He is the One who enables you to travel on land and on the sea, until when you were on the ships and We drive them with a good wind which they rejoice with, a strong gust comes to them and the waves come to them from all sides, and they think that they are (being) surrounded/encircled, they implore God faithful/loyal/devoted to His system: "If You save us from this, we will be of the thankful." [10:22]
When they ride on a ship, they call on God, faithful/loyal/devoted the system to Him. But as soon as He saves them to the shore, they set up partners. [29:65]
This is the human's state of mind (only)WHEN disasters/ difficulties come from all directions i.e. No escape (e.g. when their doctors are helpless, they will sincerely call GOD).
But for a true believer, this (GOD Alone) 'state of mind' is in his Whole life (in ALL Affairs) as commanded by the Prophet(s).