Eph 3:8 Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ;
Eph 3:8 (TR) εμοι τω ελαχιστοτερω παντων των αγιων εδοθη η χαρις αυτη εν τοις εθνεσιν ευαγγελισασθαι τον ανεξιχνιαστον πλουτον του χριστου
The greek word translated "unsearchable" is the adjective ἀνεξιχνίαστος which is found here and in Rom. 11:33 where it is translated "past finding out".
Ro 11:33 O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out <421>
The word is derived from ἀ + ἐκ + ἴχνος which litterally means "not from a footprint", and conveys the idea of untraceability. The riches PAul talks of in prison are untraceable there is no track of them in the old testament, this is magnified by the fact that these riches are linked to the revelation of "the mystery" which was hid in God Eph.3:9.
The context is also suggestive because in Romans the Gentiles receive blessing by being graft unnaturally into Israel's olive, mercy came to them but by a strict observance to retainining Israel as God's favoured and preferential nation. The Gentiles partook of Israel's blessing and hope. His ways are past finding out indeed but the national order of blessing is based on old testament revelation.
To PAul the prisoner was committed a treasure trove of riches. Eph. 1:7,18; 2:7,3:8,16; Php. 4:19; Col. 1:27; 2:2. But what is most striking about these riches is that they are directed to US the outsiders of the covenants of promise. How is this possible and under what circumstances did it take place?
PAul says this:
Eph 2:11 ¶ (AV) Wherefore remember, that ye being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands;
Eph 2:12 That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world:
There are clearly three time periods mentioned in Ephesians 2, time past (2:11), but now (2:13) and the ages to come (2:7) that is past, present and future. When PAul says in Eph. 2:12 "at that time" he is referring to the Gentiles in time past. The Gentiles were nationally estranged from God and their dark place is put in stark relation to Israel in these verses. Right division is about putting things in order and through understanding PAul and his prison ministry we receive great light and liberty from God's word.
The sad conclusion of Eph. 2:12 is that the Gentile peoples had no hope and were without God in the world. Thankfully the next verses say:
13 (AV) But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ.
14 ¶ (AV) For he is our peace, who hath made both one, and hath broken down the middle wall of partition between us;
15 (AV) Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace;
But now -- this introduces a refreshing change and new age, the "peace" here is the direct abolishing of the law of commandments contained in dogmas that symbolized Israel's advantages and the Gentiles outsidedness. We as a group of nations are not at enmity with God in the sense that we are not dispensationally disadvantaged, the salvation of God is today "sent to the Gentiles". We are not made nigh by an unnatural graft nor by some slight of hand, but rather we are a part of the "new man" freshly created by God at the end of the Acts. PAul says "for to make in himself of twain one new man, so making peace", the word "to make" here is from the word κτίζω which means to create. The Lord, from two disparate groups, one advantaged with covenants etc and the other outsiders (Gentile dogs) created in himself one new man.
This is the subject of mystery (God's secret purpose) and is past finding out in the old testament it is untraceable.