In the Iron Age only the Practice of the Naam is fruitful

When virtues decline and evil increases, the mere doing of duties and meritorious deeds bear no fruit. In such a period the perfect Masters preach the doctrine of the Name. 

When tyranny and persecution take hold of the land, when everyone is filled with gloom, when people become devoid of true strength and clear thinking, there is no easy and quick acting path save that of the Name. Good deeds bear no fruit in this age. They were a law in the past ages.

In the Iron Age, only the Lord's Name is sublime.

 Maru M5, 1075-19

Saint Ravi Das tells us that in the Golden Age, Truth was the dominant note; in the Silver Age, sacrifices; and in the Copper Age, idol worship; but in the Iron Age, it is only the Name – Naam.

In the Golden Age, Truth reigned Supreme, in the Silver Age charity, in the Copper Age, worship and piety. In the three Ages men held fast to these acts, but in the Iron Age, our mainstay is the Name – Naam.

Ravi Das, Gauri 346-10

Saint Tulsi Das holds the same view as that of Ravi Das. 

He writes:

Contemplation was the practice in the first Age, and sacrifice in the second Age. People worshipped idols of God in the Copper Age. In the Iron Age everyone is covered with dirt and impurities. And people live contented, like fish, in an ocean of sins. The Name is like the wish-fulfilling tree, and Kal, the Negative Power, is frightful.

But remembrance of the Name with the mind, cuts all the worldly ties.

Ramayana, Bal Kand 26, 2-3

In the Iron Age the Lord's Naam is the most efficacious method, and it emancipates all devotees.

Maru M4, 995-5

In the Iron Age no good act or duty is of value; no one is saved without the Naam.

Sar Bachan 681

Guru Arjan also says:

It is only the Name – Naam that saves the devotee in the Iron Age.

Bhairon M5, 1138-7

No other means is fruitful in the Iron Age. Only the practice of listening to the Name can bear fruit. Great emphasis is accordingly laid by the Gurus in the line of Guru Nanak on sowing the seed of Naam.

The Iron Age has now dawned, sow the seed of the Lord's Name. This season is not propitious for any other crop; do not be deluded by doubt.

Basant M5, 1185-7

Blessed are those who are initiated into the Name in the Iron Age.

Oh Nanak, I am a sacrifice to them who are blessed with the Lord's Name in the Iron Age.

Kama M5, 1298-14