I Guarantee it!
Heb 7:20-22 And it was not without an oath! Others became priests without any oath, but he became a priest with an oath when God said to him: "The Lord has sworn and will not change his mind: ‘You are a priest forever.’" Because of this oath, Jesus has become the guarantee of a better covenant.
Under the Law of Moses the priesthood was given by commandment, by regulation. It didn't come with guarantees. In fact God killed two of Aaron's sons because they misbehaved as priests, as also He did on other occasions. But Christ is given a guarantee. This is one aspect of the New Covenant that comes with a guarantee, and there are other guarantees associated with the New Covenant.
Those under the New Covenant are guaranteed eternal life, and they are born-again, given the Spirit of God, "who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession— to the praise of his glory." Eph 1:14 "Now it is God who makes both us and you stand firm in Christ. He anointed us, set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come." 2Cor 1:21,22
Heb 7:23-25 Now there have been many of those priests, since death prevented them from continuing in office; but because Jesus lives forever, he has a permanent priesthood. Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them.
Under the Mosaic Law there were no guarantees that you would be saved. New Covenant includes eternal security. Those who come to God through Christ will be completely saved, saved to the uttermost, saved eternally. Jesus said, "I am the gate; whoever enters through me will be saved." John 10:9a "And everyone who calls on the name of the Lord will be saved." Acts 2:21 "For if, when we were God’s enemies, we were reconciled to him through the death of his Son, how much more, having been reconciled, shall we be saved through his life!" Rom 5:10 So "Believe in the Lord Jesus, and you will be saved" Acts 16:31