as he spake by the mouth of his holy prophets, which have been since the world began: that we should be saved from our enemies, and from the hand of all that hate us; to perform the mercy promised to our fathers, and to remember his holy covenant; the oath which he sware to our father Abraham, - Luke 1:70-73

God is not only a covenant making God, He is a covenant keeping God. God promised a Savior in the garden and every promise since then pointed toward Jesus Christ.

Unlike the various religious systems of the world (and some denominations of Christianity), true Christianity is based upon the sovereignty and integrity of God not on man’s ability to keep a set of laws or maintain holiness.

The salvation of God’s children is secure because God made it sure. He spoke the promise to Abraham as recorded in the book of Genesis. The Hebrew letter makes reference to this promise in chapter 6:

For when God made promise to Abraham, because he could swear by no greater, he sware by himself, saying, Surely blessing I will bless thee, and multiplying I will multiply thee. And so, after he had patiently endured, he obtained the promise. For men verily swear by the greater: and an oath for confirmation is to them an end of all strife. Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath: that by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us: which hope we have as an anchor of the soul, both sure and stedfast, and which entereth into that within the veil; whither the forerunner is for us entered, even Jesus, made an high priest for ever after the order of Melchisedec. - Hebrews 6:13-20

The oath that God swore was with Himself for Abraham and us, not with Abraham.  This means the possibility of the oath being kept lies completely within the power of God Who is all powerful, not Abraham's faithfulness or ours.

Basically this oath means that God said He would save us; God cannot lie; therefore, God will save us.

The power and surety of it all resides in a Holy God not frail men. Therefore, we do not simply have a hope like a birthday wish for salvation.  Our hope is a sure anchor which is the faithfulness of God in Jesus Christ.