Is a son's statement to his father, "I will pray on Sunday," considered an imitation of Christians, and is it forbidden (haram)? And is his saying, "The most important thing is the intention, not the words," correct, especially when he uses the intention to justify speech that might be wrong?

The phrase "I will pray on Sunday," if intended to imitate Christians in their Sunday prayers, is forbidden, based on the Prophet's (peace be upon him) saying: "Whoever imitates a people is one of them," and "He is not of us who imitates others." One should not be complacent regarding such statements. A Muslim must be careful with their speech and keen on choosing their words, based on the Prophet's (peace be upon him) saying to Mu'adh: "Are people thrown into Hellfire upon their faces except for the harvest of their tongues?" and his saying: "Indeed, a servant may utter a word pleasing to Allah, not thinking much of it, but Allah raises him by it many degrees. And indeed, a servant may utter a word displeasing to Allah, not thinking much of it, but it casts him into Hell." Thus, a servant is held accountable for their intention, speech, and words, even if they did not believe it.