What does "having act of service justified before God" mean? I've never heard of it nor read anything about that in scripture.
James is talking about eternal judgement. Look at his first example in the context. James gives an example of a brother or sister in need. When we see that need and do not give, James asks what use is that.
15 If a brother or sister is without clothing and in need of daily food, 16 and one of you says to them, “Go in peace, be warmed and be filled,” yet you do not give them what is necessary for their body, what use is that?
In the next verse, he tells us that that is faith without works and it is dead. 17 In the same way, faith also, if it has no works, is dead, being by itself.
How do I know that this is about eternal judgement? Matthew 25. In Matthew 25, Jesus tells us of the day of judgement where He divides those for eternal life or destruction. Look at what Jesus uses as justification for each destination. The exact same thing James uses as an example, what we do or not do for someone in need.
31 “But when the Son of Man comes in His glory, and all the angels with Him, then He will sit on His glorious throne. 32 And all the nations will be gathered before Him; and He will separate them from one another, just as the shepherd separates the sheep from the goats; 33 and He will put the sheep on His right, but the goats on the left.
34 “Then the King will say to those on His right, ‘Come, you who are blessed of My Father, inherit the kingdom prepared for you from the foundation of the world. 35 For I was hungry, and you gave Me something to eat; I was thirsty, and you gave Me something to drink; I was a stranger, and you invited Me in; 36 naked, and you clothed Me; I was sick, and you visited Me; I was in prison, and you came to Me.’ 37 Then the righteous will answer Him, ‘Lord, when did we see You hungry, and feed You, or thirsty, and give You something to drink? 38 And when did we see You as a stranger, and invite You in, or naked, and clothe You? 39 And when did we see You sick, or in prison, and come to You?’ 40 And the King will answer and say to them, ‘Truly I say to you, to the extent that you did it for one of the least of these brothers or sisters of Mine, you did it for Me.’
41 “Then He will also say to those on His left, ‘Depart from Me, you accursed people, into the eternal fire which has been prepared for the devil and his angels; 42 for I was hungry, and you gave Me nothing to eat; I was thirsty, and you gave Me nothing to drink; 43 I was a stranger, and you did not invite Me in; naked, and you did not clothe Me; sick, and in prison, and you did not visit Me.’ 44 Then they themselves also will answer, ‘Lord, when did we see You hungry, or thirsty, or as a stranger, or naked, or sick, or in prison, and did not take care of You?’ 45 Then He will answer them, ‘Truly I say to you, to the extent that you did not do it for one of the least of these, you did not do it for Me, either.’ 46 These will go away into eternal punishment, but the righteous into eternal life.”