Salvation is made sure through Christ, not to all mankind, but to the Seed Rom 4:16
16 Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
In the original there is a definite article before seed:
διὰ τοῦτο ἐκ πίστεως ἵνα κατὰ χάριν εἰς τὸ εἶναι βεβαίαν τὴν ἐπαγγελίαν
παντὶ τῷ σπέρματι οὐ τῷ ἐκ τοῦ νόμου μόνον ἀλλὰ καὶ τῷ ἐκ πίστεως Ἀβραάμ ὅς ἐστιν πατὴρ πάντων ἡμῶν
The word panti all refers the seed. The word seed means:
- family, tribe, posterity,
- children, offspring, progeny
- metaph. a seed i.e. a residue, or a few survivors reserved as the germ of the next generation (just as seed is kept from the harvest for the sowing)
Its also a residue or remnant. James referred to the gentiles as a residue Acts 15:17
That the residue of men might seek after the Lord, and all the Gentiles, upon whom my name is called, saith the Lord, who doeth all these things.
Residue means left remaining.
The promise of salvation to mankind is for a particular seed, its in Christ 2 Cor 1:20
For all the promises of God in him are yea, and in him Amen, unto the glory of God by us.
The promises are sure to Christs seed Isa 53:10
10 Yet it pleased the Lord to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the Lord shall prosper in his hand.
Remember Christ is Abrahams Seed Gal 3:16 and the Church is One with Christ because Hes the Head of the Church. Just remember a seed isnt all of mankind but only a residue or remnant.