As a Christian, I believe that life should be protected from the moment of conception. Does that mean I know beyond a shadow of a doubt that the moment the sperm and egg meet there is an actual human being with a soul? No. I don't know for certain and I don't think there is a clear passage in God's Word that answers that question. A lot of my fellow Pro-Life Christians point to passages like Ps. 139, Jer. 1:5, and a bunch of other proof-texts, but the context of those passages seem to be unrelated to the exact moment that life begins. I do have a lot of questions that I plan to ask when I get to heaven. For instance, I don't understand why 50-80% (at least according to some estimates) of fertilized eggs never implant into the uterus. Are these all miniature human beings that go to heaven? I have no clue. I know that twinning often occurs during implantation (well after fertilization), so this begs the question about the point of ensoulment for twins, triplets, etc. I will not pretend to know the answers to all these difficult questions. Since I don't believe anyone can prove with absolute certainty the precise point that life begins (including when ensoulment occurs), I believe we should protect life from the moment of fertilization since there is at least a potential life there. It's always better to err on the side of life rather than destroying a potential person. That's my take on it.
BTW, abortion is not an unforgivable sin by any means. ALL sins (past, present, and future) can be totally forgiven through faith in Jesus Christ as Savior. Sometimes Pro-Lifers can be so focused on condemning women who have had abortions rather than emphasizing the grace of God that is so freely extended to even the worst of sinners (i.e. Moses, David, and the Apostle Paul had all committed murder and many other atrocious sins, but God still forgave them and used them mightily).
Knowing for sure that you have eternal life is the single most important issue in life. All people are separated from God by a sin barrier that no human effort can overcome (Isa. 64:6; Rom. 3:10, 12, 23). All human beings are born without spiritual life (Eph. 2:1). That is why Jesus Christ, who is eternal God Himself, came in human flesh so that He could suffer and die in our place - as our substitute - on the cross of Calvary. He died on the cross to pay our sin debt in FULL and rose again (1 Cor. 15:3-4). When we believe in Jesus Christ for our eternal life He has promised to give it to us:
"Most assuredly, I say to you, he who believes in Me has everlasting life" (John 6:47).
"Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life" (John 5:24).
To "believe" is simply to be convinced that it is true (Rom. 4:21) that you cannot in any way contribute to your own salvation and instead you trust Jesus alone to give you that life freely - a gift of unconditional eternal life. It means to believe His promise to give eternal life to all who believe in Him for that life. It really is that simple!
The Gospel of John is the only book written for the sole purpose of showing people how they can have eternal life (see John 20:30-31) and it mentions the word "believe" as the sole condition 98 times!
This link helped me when I was struggling with my salvation: http://faithalone.org/tracts/ycbs.html