Past and passed, while they sound the same, are not interchangeable.
Past is usually used as an adjective, and is always used before a noun. (“He was hired based on his past experience.”)
Passed is the past tense of the verb pass. (“I passed the vehicle in the left lane.”) Passed would have been the correct word choice in this context.
Aside from the misuse of the word past, what is this poet/hymn writer trying to say? Is he trying to inspire me with the word IF? IF I live till tomorrow? What makes you think I won’t make it, Bill? That, and the missing comma after tomorrow, leaves me highly uninspired.