That is, is it understandable to us why God would send His only begotten Son to earth? The answer to the question is, Yes.
When you think about it, Jesus did two things when He came to earth: He taught, and He died for our sins.
His teaching ministry, in turn, (a) elaborated on the sort of behavior God wants from us, and (b) clarified how salvation was open to all humanity. Both are very important.
On the first teaching point, I write elsewhere on this blogsite that Jesus’ elaboration “boils down to two things — having the right attitude as well as the right actions, and humbly serving others before yourself, especially those less fortunate than you are, particularly the poor — and I note that love is the key element here, as it is in throughout Jesus’ teachings.”
On the second teaching point, God’s long game was always to save all humanity (see, e.g., Gen. 12:3, 22:18, 26:4 and Gal. 3:8; see also Isaiah 49:6, quoted in Acts 13:45-47 ), and it’s interesting how often Jesus reached outside the Jewish community in his ministry (e.g., to the Samaritans and Romans) — and then, of course, there is the Great Commission (not to mention Paul’s ministry, starting with Acts 9:15).
His dying for our sins makes sense, too, and was likewise of surpassing importance. Now, I am not in this short post going to wade into a deep discussion of the theology of atonement. Instead, I’ll refer the reader to — who else? — C.S. Lewis (in his Mere Christianity, book II, chapter 4, “The Perfect Penitent”), noting that he is modest about the mechanics here, too — and also forthright in declaring its importance.