Why did God choose to display His righteousness and salvation through Jesus Christ and not in any other way?
I think before we can answer that question, another question should be addressed:
“Could God have carried out His redemptive plans apart from Jesus Christ?
Are there any other ways to accomplish what He did through Jesus Christ?
So, to rewind a bit, the issue behind all of this is what we call “Sin.” We know from the Bible that God absolutely hates sin (Psalms 5:5; Proverbs 15:9). Perhaps that is a big issue, but we have also heard that God is Love. That brings a sigh of relief because love covers a multitude of sins (1 Peter 4:8), so all God has to do is forgive us and we should be on our merry ways. But it’s not that simple - God is also Holy and Just. These attributes of God don’t function like a pie chart where God is 40% Love and 15% Holy and such; God is 100% Love, and also 100% Holy and so on. This may not quite make sense to us because we are used to seeing everything add up to a whole, and well, God isn’t bound up to neat compartments like most earthy things, but this is for another matter. Because God is 100% in everything, He cannot compromise one attribute for another.
The other issue is that sin is not simply something fallen humanity commit, but rather it is birth from sinful nature (Romans 5:12-13; Romans 6:6; Psalms 51:5). It wouldn’t make sense for me to tell a dog to stop being so “animalistic”, because a dog is an animal by nature. Perhaps an animal may exhibit human qualities but I believe at the core, much of what is done unconsciously or subconsciously is rooted in its nature. Similarly, we cannot simply enforce behavior modification to rid of our sins; it lies within the cracks of everything we think, say or do.
So if we are sinners and God hates sin and must repay evil, we are in big trouble. Fine, we might be familiar with our legal system. If God is a judge, can’t we simply post bail? The price for sin is death. We must not be confused in thinking that when we die, we have a clean slate, that the price is paid in full and the balance is zero. What we would have done is pay the price for one sin. Assuming we had committed more than one sin, we won’t get out until you paid the last penny (Matthew 5:26), and the catch is, we have nothing to pay with, being poor, pitiful, and blind (Revelations 3:17). Let’s say we take the other route and offer something or someone to die on behalf of us in our place, something that God had put in place during the Old Testament. The problem with this is that our nature remains unchanged. Again, we might be able to curb destructive behaviors like murder or adultery, but we might still secretly desire them, and if we knew there were no consequences to sin, we would go on sinning happily and shamelessly.
Some people might say, “if God can do anything, can’t He just snap His fingers and immediately take away our sins so we stop sinning?” Going down this hallway is going to lead to rooms full of passionate debaters, so to cut our steps short, if somehow God figured out how to pay the death penalty of sin for the billions of people who have walked the face of the earth, past, present, future, we run into the issue of free will. Yes, God is sovereign over everything (Psalms 115:3), but we are still responsible for the words we say and the things we do (Romans 3:5-6; Romans 14:12). Why free will cannot be jeopardized is because it is essential to love. How else can we obey the great command to love God with all our heart, soul, and strength (Deut. 6:5)? Because of our sinful nature, our will delights, inclines toward, prefers sin over God. To ask God to solve the issue by removing sin in such a way leaves injustice unsettled and humans either as robotic, programmed shells or unhappy spouses.
This leads us to the answer to the question: God chose to display His righteousness and salvation only through Jesus Christ because it does not compromise His unchanging character (Numbers 23:19; Hebrews 13:8; James 1:17), satisfies His justice and wrath (Romans 3:26; Romans 5:9), displays His rich mercy (Ephesians 2:4), eliminates human boasting and magnifies God’s glory (Romans 3:27-28; Romans 5:6), and transforms our sinful nature into a holy temple wherein God dwells (Ephesians 2:22; Romans 8:9-11). When Jesus died, the sinful nature within us died with Him (Romans 6:6). When Jesus was raised from the dead, we were also raised with Him (Romans 6:4) but with a new nature (2 Corinthians 5:17), born of Spirit (John 3:3-8). It craves pure, spiritual things (1 Peter 2:2; Romans 8:5) and no longer has thirst for wicked things (Galatians 5:16-25). This has been made possible by the Spirit of God through faith in Jesus Christ (Galatians 3:1-6). This means, when God says, “You shall love the Lord your God with all your heart and with all your soul and with all your mind.”, we respond by saying, “Gladly!” There is no twisting of the arm, no coercion, no threats muttered, because God becomes most desirable (Matthew 13:44-46). The commands of God are no longer burdensome (Matthew 11:28), but are delightful (Psalms 1:2; Psalms 119:12-16).
No other method of salvation would bring about everlasting praise and maximum glory to God than the sacrificial offering of God Himself. There has to be a song worth singing about in heaven for eternity, and there is one: Worthy is the Lamb who was slain (Revelations 5).