Interesting verses. The NAB verses are as follows:
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In him we were also chosen, destined in accord with the purpose of the one who accomplishes all things according to the intention of his will,
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so that we might exist for the praise of his glory, we who first hoped in Christ.
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In him you also, who have heard the word of truth, the gospel of your salvation, and have believed in him, were sealed with the promised holy Spirit,
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which is the first installment of our inheritance toward redemption as God’s possession, to the praise of his glory.
Verse 12: we who first hoped in Christ. Hoped.
Verse 13: notice the order - belief first, Holy Spirit second. The sealing is baptism.
Verse 14: first installment of our inheritance. This agrees with 2 Cor 1:
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12 But the one who gives us security with you in Christ and who anointed us is God;
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he has also put his seal upon us and given the Spirit in our hearts as a first installment.
Installment to an inheritance. We have the ability to inherit everlasting life. We can obviously disregard that inheritance if we so desire.
WOW Mark, and I just got done saying the NAB was OK. :) Famous last words. So we have first installment verses guarantee. Interesting. However, I think I can see where first installment comes from. Here is what Strong's says:
[for the KJV word "earnest"] NT:728 arrhabon (ar-hrab-ohn'); of Hebrew origin [OT:6162]; a pledge, i.e. part of the purchase-money or property given in advance as security for the rest: KJV - earnest.
I think "first installment" is reasonable from this. However, notice that there is a pledge. I think the Church is in error because it must think that the pledge is made by man, so it can be broken (as you said), when in fact it is clear that the pledge is made by God. (We were marked by God with the pledge in the form of the Spirit.) While man is a welcher, God is not. God's pledge is gold, so this is as good as a guarantee.